[Qemu-devel] [PATCH 2/2] pc: hack for migration compatibility from QEMU 2.0

[Paolo Bonzini]

Changing the ACPI table size causes migration to break, and the memory
hotplug work opened our eyes on how horribly we were breaking things in
2.0 already.

The ACPI table size is rounded to the next 4k, which one would think
gives some headroom.  In practice this is not the case, because the user
can control the ACPI table size (each CPU adds 105 bytes) and so some
"-smp" values will break the 4k boundary and fail to migrate.  Similarly,
PCI bridges add ~1870 bytes to the SSDT.

To fix this, hard-code 64k as the maximum ACPI table size, which
(despite being an order of magnitude smaller than 640k) should be enough
for everyone.

To fix migration from QEMU 2.0, compute the payload size of QEMU 2.0
and always use that one.  The previous patch shrunk the ACPI tables
enough that the QEMU 2.0 size should always be enough.

Non-AML tables can change depending on the configuration (especially
MADT, SRAT, HPET) but they remain the same between QEMU 2.0 and 2.1,
so we only compute our padding based on the sizes of the SSDT and DSDT.

Migration from QEMU 1.7 should work for guests that have a number of CPUs
other than 12, 13, 14, 54, 55, 56, 97, 98, 139, 140, and that have no
PCI bridges.  It was already broken from QEMU 1.7 to QEMU 2.0 in the
same way, though.

Signed-off-by: Paolo Bonzini <address@hidden>
---
 hw/i386/acpi-build.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++----
 hw/i386/pc_piix.c    | 20 +++++++++++++++++
 hw/i386/pc_q35.c     |  5 +++++
 include/hw/i386/pc.h |  1 +
 4 files changed, 83 insertions(+), 4 deletions(-)

diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
index ebc5f03..7373d93 100644
--- a/hw/i386/acpi-build.c
+++ b/hw/i386/acpi-build.c
@@ -25,7 +25,9 @@
 #include <glib.h>
 #include "qemu-common.h"
 #include "qemu/bitmap.h"
+#include "qemu/osdep.h"
 #include "qemu/range.h"
+#include "qemu/error-report.h"
 #include "hw/pci/pci.h"
 #include "qom/cpu.h"
 #include "hw/i386/pc.h"
@@ -87,6 +89,8 @@ typedef struct AcpiBuildPciBusHotplugState {
     struct AcpiBuildPciBusHotplugState *parent;
 } AcpiBuildPciBusHotplugState;
 
+unsigned bsel_alloc;
+
 static void acpi_get_dsdt(AcpiMiscInfo *info)
 {
     uint16_t *applesmc_sta;
@@ -759,8 +763,8 @@ static void *acpi_set_bsel(PCIBus *bus, void *opaque)
 static void acpi_set_pci_info(void)
 {
     PCIBus *bus = find_i440fx(); /* TODO: Q35 support */
-    unsigned bsel_alloc = 0;
 
+    assert(bsel_alloc == 0);
     if (bus) {
         /* Scan all PCI buses. Set property to enable acpi based hotplug. */
         pci_for_each_bus_depth_first(bus, acpi_set_bsel, NULL, &bsel_alloc);
@@ -1440,13 +1444,14 @@ static
 void acpi_build(PcGuestInfo *guest_info, AcpiBuildTables *tables)
 {
     GArray *table_offsets;
-    unsigned facs, dsdt, rsdt;
+    unsigned facs, ssdt, dsdt, rsdt;
     AcpiCpuInfo cpu;
     AcpiPmInfo pm;
     AcpiMiscInfo misc;
     AcpiMcfgInfo mcfg;
     PcPciInfo pci;
     uint8_t *u;
+    size_t aml_len = 0;
 
     acpi_get_cpu_info(&cpu);
     acpi_get_pm_info(&pm);
@@ -1474,13 +1479,20 @@ void acpi_build(PcGuestInfo *guest_info, 
AcpiBuildTables *tables)
     dsdt = tables->table_data->len;
     build_dsdt(tables->table_data, tables->linker, &misc);
 
+    /* Count the size of the DSDT and SSDT, we will need it for legacy
+     * sizing of ACPI tables.
+     */
+    aml_len += tables->table_data->len - dsdt;
+
     /* ACPI tables pointed to by RSDT */
     acpi_add_table(table_offsets, tables->table_data);
     build_fadt(tables->table_data, tables->linker, &pm, facs, dsdt);
 
+    ssdt = tables->table_data->len;
     acpi_add_table(table_offsets, tables->table_data);
     build_ssdt(tables->table_data, tables->linker, &cpu, &pm, &misc, &pci,
                guest_info);
+    aml_len += tables->table_data->len - ssdt;
 
     acpi_add_table(table_offsets, tables->table_data);
     build_madt(tables->table_data, tables->linker, &cpu, guest_info);
@@ -1513,12 +1525,53 @@ void acpi_build(PcGuestInfo *guest_info, 
AcpiBuildTables *tables)
     /* RSDP is in FSEG memory, so allocate it separately */
     build_rsdp(tables->rsdp, tables->linker, rsdt);
 
-    /* We'll expose it all to Guest so align size to reduce
+    /* We'll expose it all to Guest so we want to reduce
      * chance of size changes.
      * RSDP is small so it's easy to keep it immutable, no need to
      * bother with alignment.
+     *
+     * We used to align the tables to 4k, but of course this would
+     * too simple to be enough.  4k turned out to be too small an
+     * alignment very soon, and in fact it is almost impossible to
+     * keep the table size stable for all (max_cpus, max_memory_slots)
+     * combinations.  So the table size is always 64k for pc-2.1 and
+     * we give an error if the table grows beyond that limit.
+     *
+     * We still have the problem of migrating from "-M pc-2.0".  For that,
+     * we exploit the fact that QEMU 2.1 generates _smaller_ tables than 2.0
+     * and we can always pad the smaller tables with zeros.  We can then use
+     * the exact size of the 2.0 tables.
+     *
+     * All this is for PIIX4, since QEMU 2.0 didn't support Q35 migration.
      */
-    acpi_align_size(tables->table_data, 0x1000);
+    if (guest_info->legacy_acpi_table_size) {
+        /* Subtracting aml_len gives the size of fixed tables.  Then add the
+         * size of the PIIX4 DSDT/SSDT in QEMU 2.0.
+         */
+        int legacy_aml_len =
+            guest_info->legacy_acpi_table_size +
+            97 * max_cpus +
+            1875 * (MAX(bsel_alloc, 1) - 1);
+        int legacy_table_size =
+            ROUND_UP(tables->table_data->len - aml_len + legacy_aml_len, 
0x1000);
+        if (tables->table_data->len > legacy_table_size) {
+            /* -M pc-2.0 doesn't support memory hotplug, so this should never
+             * happen.
+             */
+            error_report("This configuration is not supported with -M 
pc-2.0.");
+            error_report("Please report this to address@hidden");
+            exit(1);
+        }
+        g_array_set_size(tables->table_data, legacy_table_size);
+    } else {
+        if (tables->table_data->len > 65536) {
+            /* As of QEMU 2.1, this fires with 160 VCPUs and 255 memory slots. 
 */
+            error_report("Too many maximum CPUs, NUMA nodes or memory slots.");
+            error_report("Please decrease one of these parameters.");
+            exit(1);
+        }
+        g_array_set_size(tables->table_data, 0x10000);
+    }
 
     acpi_align_size(tables->linker, 0x1000);
 
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 7081c08..4d3da20 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -61,6 +61,7 @@ static const int ide_irq[MAX_IDE_BUS] = { 14, 15 };
 
 static bool has_pci_info;
 static bool has_acpi_build = true;
+static int legacy_acpi_table_size;
 static bool smbios_defaults = true;
 static bool smbios_legacy_mode;
 /* Make sure that guest addresses aligned at 1Gbyte boundaries get mapped to
@@ -163,6 +164,7 @@ static void pc_init1(MachineState *machine,
     guest_info = pc_guest_info_init(below_4g_mem_size, above_4g_mem_size);
 
     guest_info->has_acpi_build = has_acpi_build;
+    guest_info->legacy_acpi_table_size = legacy_acpi_table_size;
 
     guest_info->has_pci_info = has_pci_info;
     guest_info->isapc_ram_fw = !pci_enabled;
@@ -297,6 +299,24 @@ static void pc_init_pci(MachineState *machine)
 
 static void pc_compat_2_0(MachineState *machine)
 {
+    /* This value depends on the actual DSDT and SSDT compiled into
+     * the source QEMU; unfortunately it depends on the binary and
+     * not on the machine type, so we cannot make pc-1.7 work on
+     * both QEMU 1.7 and QEMU 2.0.
+     *
+     * Large variations cause migration to fail for more than one
+     * consecutive value of the "-smp" maxcpus option.
+     *
+     * For small variations of the kind caused by different iasl versions,
+     * the 4k rounding usually leaves slack.  However, there could be still
+     * one or two values that break.  For QEMU 1.7 and QEMU 2.0 the
+     * slack is only ~10 bytes before one "-smp maxcpus" value breaks!
+     * on the actual contents of the DSDT and SSDT).
+     *
+     * 6652 is valid for QEMU 2.0, the right value for pc-1.7 on
+     * QEMU 1.7 it is 6414.  For RHEL/CentOS 7.0 it is 6418.
+     */
+    legacy_acpi_table_size = 6652;
     smbios_legacy_mode = true;
     has_reserved_memory = false;
 }
diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index f551961..c39ee98 100644
--- a/hw/i386/pc_q35.c
+++ b/hw/i386/pc_q35.c
@@ -155,6 +155,11 @@ static void pc_q35_init(MachineState *machine)
     guest_info->has_acpi_build = has_acpi_build;
     guest_info->has_reserved_memory = has_reserved_memory;
 
+    /* Migration was not supported in 2.0 for Q35, so do not bother
+     * with this hack (see hw/i386/acpi-build.c).
+     */
+    guest_info->legacy_acpi_table_size = 0;
+
     if (smbios_defaults) {
         MachineClass *mc = MACHINE_GET_CLASS(machine);
         /* These values are guest ABI, do not change */
diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
index 1c0c382..f4b9b2b 100644
--- a/include/hw/i386/pc.h
+++ b/include/hw/i386/pc.h
@@ -94,6 +94,7 @@ struct PcGuestInfo {
     uint64_t *node_mem;
     uint64_t *node_cpu;
     FWCfgState *fw_cfg;
+    int legacy_acpi_table_size;
     bool has_acpi_build;
     bool has_reserved_memory;
 };
-- 
1.8.3.1