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

[Laszlo Ersek]

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".

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.

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,
>