Re: [Qemu-devel] [PATCH v4 2/8] acpi: add vmcoreinfo device

[Laszlo Ersek]

On 07/14/17 21:26, Michael S. Tsirkin wrote:
> On Fri, Jul 14, 2017 at 08:20:05PM +0200, Marc-André Lureau wrote:
>> The VM coreinfo (vmcoreinfo) device is an emulated device which
>> exposes a 4k memory range to the guest to store various informations
>> useful to debug the guest OS. (it is greatly inspired by the VMGENID
>> device implementation)
>>
>> This is an early-boot alternative to the qemu-ga VMDUMP_INFO event
>> proposed in "[PATCH 00/21] WIP: dump: add kaslr support".
>>
>> A proof-of-concept kernel module:
>> https://github.com/elmarco/vmgenid-test/blob/master/qemuvmci-test.c
>>
>> Signed-off-by: Marc-André Lureau <address@hidden>
>> Reviewed-by: Laszlo Ersek <address@hidden>
> 
> It worries me that the format of this seems completely undefined
> except in the patchset cover letter.
> I don't think we should merge this before it is.

I'm not sure what you mean, this patch adds "docs/specs/vmcoreinfo.txt".
That file is the first level contract between the guest firmware
(generally, via the linker/loader), the guest kernel driver
(specifically), and QEMU (also specifically).

The second level contract is the guest kernel's vmcoreinfo ELF note
(which is pointed-to by the first level contract). The layout of that is
specified elsewhere indeed (I don't think it belongs here).

We've taken care not to trust anything coming from the guest kernel.

Can you clarify please?

Thanks
Laszlo

> 
>> ---
>>  include/hw/acpi/aml-build.h        |   1 +
>>  include/hw/acpi/vmcoreinfo.h       |  37 +++++++
>>  hw/acpi/aml-build.c                |   2 +
>>  hw/acpi/vmcoreinfo.c               | 208 
>> +++++++++++++++++++++++++++++++++++++
>>  hw/i386/acpi-build.c               |  14 +++
>>  default-configs/arm-softmmu.mak    |   1 +
>>  default-configs/i386-softmmu.mak   |   1 +
>>  default-configs/x86_64-softmmu.mak |   1 +
>>  docs/specs/vmcoreinfo.txt          | 138 ++++++++++++++++++++++++
>>  hw/acpi/Makefile.objs              |   1 +
>>  10 files changed, 404 insertions(+)
>>  create mode 100644 include/hw/acpi/vmcoreinfo.h
>>  create mode 100644 hw/acpi/vmcoreinfo.c
>>  create mode 100644 docs/specs/vmcoreinfo.txt
>>
>> diff --git a/include/hw/acpi/aml-build.h b/include/hw/acpi/aml-build.h
>> index 88d0738d76..cf781bcd34 100644
>> --- a/include/hw/acpi/aml-build.h
>> +++ b/include/hw/acpi/aml-build.h
>> @@ -211,6 +211,7 @@ struct AcpiBuildTables {
>>      GArray *rsdp;
>>      GArray *tcpalog;
>>      GArray *vmgenid;
>> +    GArray *vmcoreinfo;
>>      BIOSLinker *linker;
>>  } AcpiBuildTables;
>>  
>> diff --git a/include/hw/acpi/vmcoreinfo.h b/include/hw/acpi/vmcoreinfo.h
>> new file mode 100644
>> index 0000000000..6a73bcd1b2
>> --- /dev/null
>> +++ b/include/hw/acpi/vmcoreinfo.h
>> @@ -0,0 +1,37 @@
>> +#ifndef ACPI_VMCOREINFO_H
>> +#define ACPI_VMCOREINFO_H
>> +
>> +#include "hw/acpi/bios-linker-loader.h"
>> +#include "hw/qdev.h"
>> +
>> +#define VMCOREINFO_DEVICE           "vmcoreinfo"
>> +#define VMCOREINFO_FW_CFG_FILE      "etc/vmcoreinfo"
>> +#define VMCOREINFO_ADDR_FW_CFG_FILE "etc/vmcoreinfo-addr"
>> +
>> +/* Occupy a page of memory */
>> +#define VMCOREINFO_FW_CFG_SIZE      4096
>> +
>> +/* allow space for OVMF SDT Header Probe Supressor */
>> +#define VMCOREINFO_OFFSET           sizeof(AcpiTableHeader)
>> +
>> +#define VMCOREINFO(obj) OBJECT_CHECK(VMCoreInfoState, (obj), 
>> VMCOREINFO_DEVICE)
>> +
>> +typedef struct VMCoreInfoState {
>> +    DeviceClass parent_obj;
>> +    uint8_t vmcoreinfo_addr_le[8];   /* Address of memory region */
>> +    bool write_pointer_available;
>> +} VMCoreInfoState;
>> +
>> +/* returns NULL unless there is exactly one device */
>> +static inline Object *find_vmcoreinfo_dev(void)
>> +{
>> +    return object_resolve_path_type("", VMCOREINFO_DEVICE, NULL);
>> +}
>> +
>> +void vmcoreinfo_build_acpi(VMCoreInfoState *vis, GArray *table_data,
>> +                           GArray *vmci, BIOSLinker *linker);
>> +void vmcoreinfo_add_fw_cfg(VMCoreInfoState *vis, FWCfgState *s, GArray 
>> *vmci);
>> +bool vmcoreinfo_get(VMCoreInfoState *vis, uint64_t *paddr, uint32_t *size,
>> +                    Error **errp);
>> +
>> +#endif
>> diff --git a/hw/acpi/aml-build.c b/hw/acpi/aml-build.c
>> index 36a6cc450e..47043ade4a 100644
>> --- a/hw/acpi/aml-build.c
>> +++ b/hw/acpi/aml-build.c
>> @@ -1561,6 +1561,7 @@ void acpi_build_tables_init(AcpiBuildTables *tables)
>>      tables->table_data = g_array_new(false, true /* clear */, 1);
>>      tables->tcpalog = g_array_new(false, true /* clear */, 1);
>>      tables->vmgenid = g_array_new(false, true /* clear */, 1);
>> +    tables->vmcoreinfo = g_array_new(false, true /* clear */, 1);
>>      tables->linker = bios_linker_loader_init();
>>  }
>>  
>> @@ -1571,6 +1572,7 @@ void acpi_build_tables_cleanup(AcpiBuildTables 
>> *tables, bool mfre)
>>      g_array_free(tables->table_data, true);
>>      g_array_free(tables->tcpalog, mfre);
>>      g_array_free(tables->vmgenid, mfre);
>> +    g_array_free(tables->vmcoreinfo, mfre);
>>  }
>>  
>>  /* Build rsdt table */
>> diff --git a/hw/acpi/vmcoreinfo.c b/hw/acpi/vmcoreinfo.c
>> new file mode 100644
>> index 0000000000..0ea41de8d9
>> --- /dev/null
>> +++ b/hw/acpi/vmcoreinfo.c
>> @@ -0,0 +1,208 @@
>> +/*
>> + *  Virtual Machine coreinfo device
>> + *  (based on Virtual Machine Generation ID Device)
>> + *
>> + *  Copyright (C) 2017 Red Hat, Inc.
>> + *  Copyright (C) 2017 Skyport Systems.
>> + *
>> + *  Authors: Marc-André Lureau <address@hidden>
>> + *           Ben Warren <address@hidden>
>> + *
>> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
>> + * See the COPYING file in the top-level directory.
>> + *
>> + */
>> +#include "qemu/osdep.h"
>> +#include "hw/acpi/acpi.h"
>> +#include "hw/acpi/aml-build.h"
>> +#include "hw/acpi/vmcoreinfo.h"
>> +#include "hw/nvram/fw_cfg.h"
>> +#include "sysemu/sysemu.h"
>> +#include "qapi/error.h"
>> +
>> +void vmcoreinfo_build_acpi(VMCoreInfoState *vis, GArray *table_data,
>> +                           GArray *vmci, BIOSLinker *linker)
>> +{
>> +    Aml *ssdt, *dev, *scope, *method, *addr, *if_ctx;
>> +    uint32_t vcia_offset;
>> +
>> +    g_array_set_size(vmci, VMCOREINFO_FW_CFG_SIZE);
>> +
>> +    /* Put this in a separate SSDT table */
>> +    ssdt = init_aml_allocator();
>> +
>> +    /* Reserve space for header */
>> +    acpi_data_push(ssdt->buf, sizeof(AcpiTableHeader));
>> +
>> +    /* Storage address */
>> +    vcia_offset = table_data->len +
>> +        build_append_named_dword(ssdt->buf, "VCIA");
>> +    scope = aml_scope("\\_SB");
>> +    dev = aml_device("VMCI");
>> +    aml_append(dev, aml_name_decl("_HID", aml_string("QEMUVMCI")));
>> +
>> +    /* Simple status method to check that address is linked and non-zero */
>> +    method = aml_method("_STA", 0, AML_NOTSERIALIZED);
>> +    addr = aml_local(0);
>> +    aml_append(method, aml_store(aml_int(0xf), addr));
>> +    if_ctx = aml_if(aml_equal(aml_name("VCIA"), aml_int(0)));
>> +    aml_append(if_ctx, aml_store(aml_int(0), addr));
>> +    aml_append(method, if_ctx);
>> +    aml_append(method, aml_return(addr));
>> +    aml_append(dev, method);
>> +
>> +    /* the ADDR method returns two 32-bit words representing the lower and
>> +     * upper halves of the physical address of the vmcoreinfo area
>> +     */
>> +    method = aml_method("ADDR", 0, AML_NOTSERIALIZED);
>> +
>> +    addr = aml_local(0);
>> +    aml_append(method, aml_store(aml_package(2), addr));
>> +
>> +    aml_append(method, aml_store(aml_add(aml_name("VCIA"),
>> +                                         aml_int(VMCOREINFO_OFFSET), NULL),
>> +                                 aml_index(addr, aml_int(0))));
>> +    aml_append(method, aml_store(aml_int(0), aml_index(addr, aml_int(1))));
>> +    aml_append(method, aml_return(addr));
>> +
>> +    aml_append(dev, method);
>> +    aml_append(scope, dev);
>> +    aml_append(ssdt, scope);
>> +
>> +    g_array_append_vals(table_data, ssdt->buf->data, ssdt->buf->len);
>> +
>> +    /* Allocate guest memory */
>> +    bios_linker_loader_alloc(linker, VMCOREINFO_FW_CFG_FILE, vmci, 4096,
>> +                             false /* page boundary, high memory */);
>> +
>> +    /* Patch address of vmcoreinfo fw_cfg blob into the ADDR fw_cfg
>> +     * blob so QEMU can read the info from there.  The address is
>> +     * expected to be < 4GB, but write 64 bits anyway.
>> +     * The address that is patched in is offset in order to implement
>> +     * the "OVMF SDT Header probe suppressor"
>> +     * see docs/specs/vmcoreinfo.txt for more details.
>> +     */
>> +    bios_linker_loader_write_pointer(linker,
>> +        VMCOREINFO_ADDR_FW_CFG_FILE, 0, sizeof(uint64_t),
>> +        VMCOREINFO_FW_CFG_FILE, VMCOREINFO_OFFSET);
>> +
>> +    /* Patch address of vmcoreinfo into the AML so OSPM can retrieve
>> +     * and read it.  Note that while we provide storage for 64 bits, only
>> +     * the least-signficant 32 get patched into AML.
>> +     */
>> +    bios_linker_loader_add_pointer(linker,
>> +        ACPI_BUILD_TABLE_FILE, vcia_offset, sizeof(uint32_t),
>> +        VMCOREINFO_FW_CFG_FILE, 0);
>> +
>> +    build_header(linker, table_data,
>> +        (void *)(table_data->data + table_data->len - ssdt->buf->len),
>> +        "SSDT", ssdt->buf->len, 1, NULL, "VMCOREIN");
>> +    free_aml_allocator();
>> +}
>> +
>> +void vmcoreinfo_add_fw_cfg(VMCoreInfoState *vis, FWCfgState *s, GArray 
>> *vmci)
>> +{
>> +    /* Create a read-only fw_cfg file for vmcoreinfo allocation */
>> +    /* XXX: linker could learn to allocate without backing fw_cfg? */
>> +    fw_cfg_add_file(s, VMCOREINFO_FW_CFG_FILE, vmci->data,
>> +                    VMCOREINFO_FW_CFG_SIZE);
>> +    /* Create a read-write fw_cfg file for Address */
>> +    fw_cfg_add_file_callback(s, VMCOREINFO_ADDR_FW_CFG_FILE, NULL, NULL,
>> +                             vis->vmcoreinfo_addr_le,
>> +                             ARRAY_SIZE(vis->vmcoreinfo_addr_le), false);
>> +}
>> +
>> +bool vmcoreinfo_get(VMCoreInfoState *vis,
>> +                    uint64_t *paddr, uint32_t *size,
>> +                    Error **errp)
>> +{
>> +    uint32_t vmcoreinfo_addr;
>> +    uint32_t version;
>> +
>> +    assert(vis);
>> +    assert(paddr);
>> +    assert(size);
>> +
>> +    memcpy(&vmcoreinfo_addr, vis->vmcoreinfo_addr_le, 
>> sizeof(vmcoreinfo_addr));
>> +    vmcoreinfo_addr = le32_to_cpu(vmcoreinfo_addr);
>> +    if (!vmcoreinfo_addr) {
>> +        error_setg(errp, "BIOS has not yet written the address of %s",
>> +                   VMCOREINFO_DEVICE);
>> +        return false;
>> +    }
>> +
>> +    cpu_physical_memory_read(vmcoreinfo_addr, &version, sizeof(version));
>> +    if (version != 0) {
>> +        error_setg(errp, "Unknown %s memory version", VMCOREINFO_DEVICE);
>> +        return false;
>> +    }
>> +
>> +    cpu_physical_memory_read(vmcoreinfo_addr + 4, paddr, sizeof(*paddr));
>> +    *paddr = le64_to_cpu(*paddr);
>> +    cpu_physical_memory_read(vmcoreinfo_addr + 12, size, sizeof(*size));
>> +    *size = le32_to_cpu(*size);
>> +
>> +    return true;
>> +}
>> +
>> +static const VMStateDescription vmstate_vmcoreinfo = {
>> +    .name = "vmcoreinfo",
>> +    .version_id = 1,
>> +    .minimum_version_id = 1,
>> +    .fields = (VMStateField[]) {
>> +        VMSTATE_UINT8_ARRAY(vmcoreinfo_addr_le, VMCoreInfoState, 
>> sizeof(uint64_t)),
>> +        VMSTATE_END_OF_LIST()
>> +    },
>> +};
>> +
>> +static void vmcoreinfo_handle_reset(void *opaque)
>> +{
>> +    VMCoreInfoState *vis = VMCOREINFO(opaque);
>> +
>> +    /* Clear the guest-allocated address when the VM resets */
>> +    memset(vis->vmcoreinfo_addr_le, 0, ARRAY_SIZE(vis->vmcoreinfo_addr_le));
>> +}
>> +
>> +static void vmcoreinfo_realize(DeviceState *dev, Error **errp)
>> +{
>> +    if (!bios_linker_loader_can_write_pointer()) {
>> +        error_setg(errp, "%s requires DMA write support in fw_cfg, "
>> +                   "which this machine type does not provide",
>> +                   VMCOREINFO_DEVICE);
>> +        return;
>> +    }
>> +
>> +    /* Given that this function is executing, there is at least one 
>> VMCOREINFO
>> +     * device. Check if there are several.
>> +     */
>> +    if (!find_vmcoreinfo_dev()) {
>> +        error_setg(errp, "at most one %s device is permitted",
>> +                   VMCOREINFO_DEVICE);
>> +        return;
>> +    }
>> +
>> +    qemu_register_reset(vmcoreinfo_handle_reset, dev);
>> +}
>> +
>> +static void vmcoreinfo_device_class_init(ObjectClass *klass, void *data)
>> +{
>> +    DeviceClass *dc = DEVICE_CLASS(klass);
>> +
>> +    dc->vmsd = &vmstate_vmcoreinfo;
>> +    dc->realize = vmcoreinfo_realize;
>> +    dc->hotpluggable = false;
>> +}
>> +
>> +static const TypeInfo vmcoreinfo_device_info = {
>> +    .name          = VMCOREINFO_DEVICE,
>> +    .parent        = TYPE_DEVICE,
>> +    .instance_size = sizeof(VMCoreInfoState),
>> +    .class_init    = vmcoreinfo_device_class_init,
>> +};
>> +
>> +static void vmcoreinfo_register_types(void)
>> +{
>> +    type_register_static(&vmcoreinfo_device_info);
>> +}
>> +
>> +type_init(vmcoreinfo_register_types)
>> diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
>> index 6b7bade183..7ac529680e 100644
>> --- a/hw/i386/acpi-build.c
>> +++ b/hw/i386/acpi-build.c
>> @@ -43,6 +43,7 @@
>>  #include "sysemu/tpm.h"
>>  #include "hw/acpi/tpm.h"
>>  #include "hw/acpi/vmgenid.h"
>> +#include "hw/acpi/vmcoreinfo.h"
>>  #include "sysemu/tpm_backend.h"
>>  #include "hw/timer/mc146818rtc_regs.h"
>>  #include "sysemu/numa.h"
>> @@ -2631,6 +2632,7 @@ void acpi_build(AcpiBuildTables *tables, MachineState 
>> *machine)
>>      GArray *tables_blob = tables->table_data;
>>      AcpiSlicOem slic_oem = { .id = NULL, .table_id = NULL };
>>      Object *vmgenid_dev;
>> +    Object *vmcoreinfo_dev;
>>  
>>      acpi_get_pm_info(&pm);
>>      acpi_get_misc_info(&misc);
>> @@ -2680,6 +2682,12 @@ void acpi_build(AcpiBuildTables *tables, MachineState 
>> *machine)
>>          vmgenid_build_acpi(VMGENID(vmgenid_dev), tables_blob,
>>                             tables->vmgenid, tables->linker);
>>      }
>> +    vmcoreinfo_dev = find_vmcoreinfo_dev();
>> +    if (vmcoreinfo_dev) {
>> +        acpi_add_table(table_offsets, tables_blob);
>> +        vmcoreinfo_build_acpi(VMCOREINFO(vmcoreinfo_dev), tables_blob,
>> +                              tables->vmcoreinfo, tables->linker);
>> +    }
>>  
>>      if (misc.has_hpet) {
>>          acpi_add_table(table_offsets, tables_blob);
>> @@ -2856,6 +2864,7 @@ void acpi_setup(void)
>>      AcpiBuildTables tables;
>>      AcpiBuildState *build_state;
>>      Object *vmgenid_dev;
>> +    Object *vmcoreinfo_dev;
>>  
>>      if (!pcms->fw_cfg) {
>>          ACPI_BUILD_DPRINTF("No fw cfg. Bailing out.\n");
>> @@ -2897,6 +2906,11 @@ void acpi_setup(void)
>>          vmgenid_add_fw_cfg(VMGENID(vmgenid_dev), pcms->fw_cfg,
>>                             tables.vmgenid);
>>      }
>> +    vmcoreinfo_dev = find_vmcoreinfo_dev();
>> +    if (vmcoreinfo_dev) {
>> +        vmcoreinfo_add_fw_cfg(VMCOREINFO(vmcoreinfo_dev), pcms->fw_cfg,
>> +                              tables.vmcoreinfo);
>> +    }
>>  
>>      if (!pcmc->rsdp_in_ram) {
>>          /*
>> diff --git a/default-configs/arm-softmmu.mak 
>> b/default-configs/arm-softmmu.mak
>> index 93e995d318..320dd3680d 100644
>> --- a/default-configs/arm-softmmu.mak
>> +++ b/default-configs/arm-softmmu.mak
>> @@ -120,6 +120,7 @@ CONFIG_XIO3130=y
>>  CONFIG_IOH3420=y
>>  CONFIG_I82801B11=y
>>  CONFIG_ACPI=y
>> +CONFIG_ACPI_VMCOREINFO=y
>>  CONFIG_SMBIOS=y
>>  CONFIG_ASPEED_SOC=y
>>  CONFIG_GPIO_KEY=y
>> diff --git a/default-configs/i386-softmmu.mak 
>> b/default-configs/i386-softmmu.mak
>> index d2ab2f6655..df68628895 100644
>> --- a/default-configs/i386-softmmu.mak
>> +++ b/default-configs/i386-softmmu.mak
>> @@ -59,3 +59,4 @@ CONFIG_SMBIOS=y
>>  CONFIG_HYPERV_TESTDEV=$(CONFIG_KVM)
>>  CONFIG_PXB=y
>>  CONFIG_ACPI_VMGENID=y
>> +CONFIG_ACPI_VMCOREINFO=y
>> diff --git a/default-configs/x86_64-softmmu.mak 
>> b/default-configs/x86_64-softmmu.mak
>> index 9bde2f1c4b..e39ad5a680 100644
>> --- a/default-configs/x86_64-softmmu.mak
>> +++ b/default-configs/x86_64-softmmu.mak
>> @@ -59,3 +59,4 @@ CONFIG_SMBIOS=y
>>  CONFIG_HYPERV_TESTDEV=$(CONFIG_KVM)
>>  CONFIG_PXB=y
>>  CONFIG_ACPI_VMGENID=y
>> +CONFIG_ACPI_VMCOREINFO=y
>> diff --git a/docs/specs/vmcoreinfo.txt b/docs/specs/vmcoreinfo.txt
>> new file mode 100644
>> index 0000000000..36d5a39ab1
>> --- /dev/null
>> +++ b/docs/specs/vmcoreinfo.txt
>> @@ -0,0 +1,138 @@
>> +VIRTUAL MACHINE COREINFO DEVICE
>> +===============================
>> +
>> +Copyright (C) 2017 Red Hat, Inc.
>> +
>> +This work is licensed under the terms of the GNU GPL, version 2 or later.
>> +See the COPYING file in the top-level directory.
>> +
>> +===
>> +
>> +The VM coreinfo (vmcoreinfo) device is an emulated device which
>> +exposes a 4k memory range to the guest to store various informations
>> +useful to debug the guest OS.
>> +
>> +QEMU Implementation
>> +-------------------
>> +
>> +The vmcoreinfo device is put in its own ACPI descriptor table, in a
>> +Secondary System Description Table, or SSDT.
>> +
>> +The following is a dump of the contents from a running system:
>> +
>> +# iasl -p ./SSDT -d /sys/firmware/acpi/tables/SSDT
>> +/*
>> + * Intel ACPI Component Architecture
>> + * AML/ASL+ Disassembler version 20160831-64
>> + * Copyright (c) 2000 - 2016 Intel Corporation
>> + *
>> + * Disassembling to symbolic ASL+ operators
>> + *
>> + * Disassembly of /sys/firmware/acpi/tables/SSDT, Mon Apr 24 15:59:53 2017
>> + *
>> + * Original Table Header:
>> + *     Signature        "SSDT"
>> + *     Length           0x00000086 (134)
>> + *     Revision         0x01
>> + *     Checksum         0x5C
>> + *     OEM ID           "BOCHS "
>> + *     OEM Table ID     "VMCOREIN"
>> + *     OEM Revision     0x00000001 (1)
>> + *     Compiler ID      "BXPC"
>> + *     Compiler Version 0x00000001 (1)
>> + */
>> +DefinitionBlock ("", "SSDT", 1, "BOCHS ", "VMCOREIN", 0x00000001)
>> +{
>> +    Name (VCIA, 0x3FFFF000)
>> +    Scope (\_SB)
>> +    {
>> +        Device (VMCI)
>> +        {
>> +            Name (_HID, "QEMUVMCI")  // _HID: Hardware ID
>> +            Method (_STA, 0, NotSerialized)  // _STA: Status
>> +            {
>> +                Local0 = 0x0F
>> +                If (VCIA == Zero)
>> +                {
>> +                    Local0 = Zero
>> +                }
>> +
>> +                Return (Local0)
>> +            }
>> +
>> +            Method (ADDR, 0, NotSerialized)
>> +            {
>> +                Local0 = Package (0x02) {}
>> +                Local0 [Zero] = (VCIA + 0x24)
>> +                Local0 [One] = Zero
>> +                Return (Local0)
>> +            }
>> +        }
>> +    }
>> +}
>> +
>> +
>> +Design Details:
>> +---------------
>> +
>> +QEMU must be able to read the contents of the device memory,
>> +specifically when starting a memory dump.  In order to do this, QEMU
>> +must know the address that has been allocated.
>> +
>> +The mechanism chosen for this memory sharing is writeable fw_cfg blobs.
>> +These are data object that are visible to both QEMU and guests, and are
>> +addressable as sequential files.
>> +
>> +More information about fw_cfg can be found in "docs/specs/fw_cfg.txt"
>> +
>> +Two fw_cfg blobs are used in this case:
>> +
>> +/etc/vmcoreinfo      - used to allocate memory range, read-only to the guest
>> +/etc/vmcoreinfo-addr - contains the address of the allocated range
>> +                     - writeable by the guest
>> +
>> +
>> +QEMU sends the following commands to the guest at startup:
>> +
>> +1. Allocate memory for vmcoreinfo fw_cfg blob.
>> +2. Write the address of vmcoreinfo into the SSDT (VCIA ACPI variable as
>> +   shown above in the iasl dump).  Note that this change is not propagated
>> +   back to QEMU.
>> +3. Write the address of vmcoreinfo back to QEMU's copy of vmcoreinfo-addr
>> +   via the fw_cfg DMA interface.
>> +
>> +After step 3, QEMU is able to read the contents of vmcoreinfo.
>> +
>> +The value of VCIA is persisted via the VMState mechanism.
>> +
>> +
>> +Storage Format:
>> +---------------
>> +
>> +The content is expected to use little-endian format.
>> +
>> +In order to implement an OVMF "SDT Header Probe Suppressor", the contents of
>> +the vmcoreinfo blob has 36 bytes of padding:
>> +
>> ++-----------------------------------+
>> +| SSDT with OEM Table ID = VMCOREIN |
>> ++-----------------------------------+
>> +| ...                               |       TOP OF PAGE
>> +| VCIA dword object ----------------|-----> +---------------------------+
>> +| ...                               |       | fw-allocated array for    |
>> +| _STA method referring to VCIA     |       | "etc/vmcoreinfo"          |
>> +| ...                               |       +---------------------------+
>> +| ADDR method referring to VCIA     |       |  0: OVMF SDT Header probe |
>> +| ...                               |       |     suppressor            |
>> ++-----------------------------------+       | 36: uint32 version field  |
>> +                                            | 40: info contents         |
>> +                                            |     ....                  |
>> +                                            +---------------------------+
>> +                                            END OF PAGE
>> +
>> +Version 0 content:
>> +
>> + uint64 paddr:
>> +  Physical address of the Linux vmcoreinfo ELF note.
>> + uint32 size:
>> +  Size of the vmcoreinfo ELF note.
>> diff --git a/hw/acpi/Makefile.objs b/hw/acpi/Makefile.objs
>> index 11c35bcb44..9623078f95 100644
>> --- a/hw/acpi/Makefile.objs
>> +++ b/hw/acpi/Makefile.objs
>> @@ -6,6 +6,7 @@ common-obj-$(CONFIG_ACPI_MEMORY_HOTPLUG) += memory_hotplug.o
>>  common-obj-$(CONFIG_ACPI_CPU_HOTPLUG) += cpu.o
>>  common-obj-$(CONFIG_ACPI_NVDIMM) += nvdimm.o
>>  common-obj-$(CONFIG_ACPI_VMGENID) += vmgenid.o
>> +common-obj-$(CONFIG_ACPI_VMCOREINFO) += vmcoreinfo.o
>>  common-obj-$(call lnot,$(CONFIG_ACPI_X86)) += acpi-stub.o
>>  
>>  common-obj-y += acpi_interface.o
>> -- 
>> 2.13.1.395.gf7b71de06