Re: [Qemu-devel] [QEMU] [PATCH 7/8] bootdevice: FW_CFG interface for LCHS values

[Sam Eiderman]

> On 12 Jun 2019, at 15:27, Laszlo Ersek <address@hidden> wrote:
> 
> On 06/12/19 11:42, Sam Eiderman wrote:
>> Using fw_cfg, supply logical CHS values directly from QEMU to the BIOS.
>> 
>> Non-standard logical geometries break under QEMU.
>> 
>> A virtual disk which contains an operating system which depends on
>> logical geometries (consistent values being reported from BIOS INT13
>> AH=08) will most likely break under QEMU/SeaBIOS if it has non-standard
>> logical geometries - for example 56 SPT (sectors per track).
>> No matter what QEMU will report - SeaBIOS, for large enough disks - will
>> use LBA translation, which will report 63 SPT instead.
>> 
>> In addition we cannot force SeaBIOS to rely on physical geometries at
>> all. A virtio-blk-pci virtual disk with 255 phyiscal heads cannot
>> report more than 16 physical heads when moved to an IDE controller,
>> since the ATA spec allows a maximum of 16 heads - this is an artifact of
>> virtualization.
>> 
>> By supplying the logical geometries directly we are able to support such
>> "exotic" disks.
>> 
>> We serialize this information in a similar way to the "bootorder"
>> interface.
>> The fw_cfg entry is "bootdevices" and it serializes a struct.
>> At the moment the struct holds the values of logical CHS values but it
>> can be expanded easily due to the extendable ABI implemented.
>> 
>> (In the future, we can pass the bootindex through "bootdevices" instead
>> "bootorder" - unifying all bootdevice information in one fw_cfg value)
> 
> I would disagree with that. UEFI guest firmware doesn't seem to have any
> use for this new type of information ("logical CHS values"), so the
> current interface (the "bootorder" fw_cfg file) should continue to work.
> The ArmVirtQemu and OVMF platform firmwares (built from the edk2
> project, and bundled with QEMU 4.1+) implement some serious parsing and
> processing for "bootorder”.

I agree, I didn’t mean to say that “bootdevices" will replace “bootorder”,
they will have to reside side by side.
I just meant to emphasis that bootorder is not extendible - adding more disk
specific fields other than bootorder (that for some platforms will be unused)
is not possible. “bootdevices” will work for LCHS, if another entry has to be
passed - it can be added to “bootdevice”.
Migrating “bootorder” into a different fw_cfg value is a tedious, probably not
worth it, effort.

> 
> Independently, another comment:
> 
>> The PV interface through fw_cfg could have also been implemented using
>> device specific keys, e.g.: "/etc/bootdevice/%s/logical_geometry" where
>> %s is the device name QEMU produces - but this implementation would
>> require much more code refactoring, both in QEMU and SeaBIOS, so the
>> current implementation was chosen.
>> 
>> Reviewed-by: Karl Heubaum <address@hidden>
>> Reviewed-by: Arbel Moshe <address@hidden>
>> Signed-off-by: Sam Eiderman <address@hidden>
>> ---
>> bootdevice.c            | 42 ++++++++++++++++++++++++++++++++++++++++++
>> hw/nvram/fw_cfg.c       | 14 +++++++++++---
>> include/sysemu/sysemu.h |  1 +
>> 3 files changed, 54 insertions(+), 3 deletions(-)
>> 
>> diff --git a/bootdevice.c b/bootdevice.c
>> index 2b12fb85a4..84c2a83f25 100644
>> --- a/bootdevice.c
>> +++ b/bootdevice.c
>> @@ -405,3 +405,45 @@ void del_boot_device_lchs(DeviceState *dev, const char 
>> *suffix)
>>         }
>>     }
>> }
>> +
>> +typedef struct QEMU_PACKED BootDeviceEntrySerialized {
>> +    /* Do not change field order - add new fields below */
>> +    uint32_t lcyls;
>> +    uint32_t lheads;
>> +    uint32_t lsecs;
>> +} BootDeviceEntrySerialized;
>> +
>> +/* Serialized as: struct size (4) + (device name\0 + device struct) x 
>> devices */
>> +char *get_boot_devices_info(size_t *size)
>> +{
>> +    FWLCHSEntry *i;
>> +    BootDeviceEntrySerialized s;
>> +    size_t total = 0;
>> +    char *list = NULL;
>> +
>> +    list = g_malloc0(sizeof(uint32_t));
>> +    *((uint32_t *)list) = (uint32_t)sizeof(s);
>> +    total = sizeof(uint32_t);
>> +
>> +    QTAILQ_FOREACH(i, &fw_lchs, link) {
>> +        char *bootpath;
>> +        size_t len;
>> +
>> +        bootpath = get_boot_device_path(i->dev, false, i->suffix);
>> +        s.lcyls = i->lcyls;
>> +        s.lheads = i->lheads;
>> +        s.lsecs = i->lsecs;
> 
> You should document the endianness of the fields in
> BootDeviceEntrySerialized, and then call byte order conversion functions
> here accordingly (most probably cpu_to_le32()).
> 
> As written, this code would break if you ran qemu-system-x86_64 /
> qemu-system-i386 (with TCG acceleration) on a big endian host.

Nice catch, thanks!

> 
> Thanks
> Laszlo
> 
>> +
>> +        len = strlen(bootpath) + 1;
>> +        list = g_realloc(list, total + len + sizeof(s));
>> +        memcpy(&list[total], bootpath, len);
>> +        memcpy(&list[total + len], &s, sizeof(s));
>> +        total += len + sizeof(s);
>> +
>> +        g_free(bootpath);
>> +    }
>> +
>> +    *size = total;
>> +
>> +    return list;
>> +}
>> diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
>> index 9f7b7789bc..008b21542f 100644
>> --- a/hw/nvram/fw_cfg.c
>> +++ b/hw/nvram/fw_cfg.c
>> @@ -916,13 +916,21 @@ void *fw_cfg_modify_file(FWCfgState *s, const char 
>> *filename,
>> 
>> static void fw_cfg_machine_reset(void *opaque)
>> {
>> +    MachineClass *mc = MACHINE_GET_CLASS(qdev_get_machine());
>> +    FWCfgState *s = opaque;
>>     void *ptr;
>>     size_t len;
>> -    FWCfgState *s = opaque;
>> -    char *bootindex = get_boot_devices_list(&len);
>> +    char *buf;
>> 
>> -    ptr = fw_cfg_modify_file(s, "bootorder", (uint8_t *)bootindex, len);
>> +    buf = get_boot_devices_list(&len);
>> +    ptr = fw_cfg_modify_file(s, "bootorder", (uint8_t *)buf, len);
>>     g_free(ptr);
>> +
>> +    if (!mc->legacy_fw_cfg_order) {
>> +        buf = get_boot_devices_info(&len);
>> +        ptr = fw_cfg_modify_file(s, "bootdevices", (uint8_t *)buf, len);
>> +        g_free(ptr);
>> +    }
>> }
>> 
>> static void fw_cfg_machine_ready(struct Notifier *n, void *data)
>> diff --git a/include/sysemu/sysemu.h b/include/sysemu/sysemu.h
>> index 173dfbb539..f0552006f4 100644
>> --- a/include/sysemu/sysemu.h
>> +++ b/include/sysemu/sysemu.h
>> @@ -174,6 +174,7 @@ void validate_bootdevices(const char *devices, Error 
>> **errp);
>> void add_boot_device_lchs(DeviceState *dev, const char *suffix,
>>                           uint32_t lcyls, uint32_t lheads, uint32_t lsecs);
>> void del_boot_device_lchs(DeviceState *dev, const char *suffix);
>> +char *get_boot_devices_info(size_t *size);
>> 
>> /* handler to set the boot_device order for a specific type of MachineClass 
>> */
>> typedef void QEMUBootSetHandler(void *opaque, const char *boot_order,