Re: [Qemu-devel] [QEMU] [PATCH v4 7/8] bootdevice: FW_CFG interface for LCHS values

[Gerd Hoffmann]

On Wed, Jun 19, 2019 at 12:29:04PM +0300, 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 new fw_cfg entry is "bios-geometry".
> 
> Reviewed-by: Karl Heubaum <address@hidden>
> Reviewed-by: Arbel Moshe <address@hidden>
> Signed-off-by: Sam Eiderman <address@hidden>
> ---
>  bootdevice.c            | 32 ++++++++++++++++++++++++++++++++
>  hw/nvram/fw_cfg.c       | 14 +++++++++++---
>  include/sysemu/sysemu.h |  1 +
>  3 files changed, 44 insertions(+), 3 deletions(-)
> 
> diff --git a/bootdevice.c b/bootdevice.c
> index 2b12fb85a4..b034ad7bdc 100644
> --- a/bootdevice.c
> +++ b/bootdevice.c
> @@ -405,3 +405,35 @@ void del_boot_device_lchs(DeviceState *dev, const char 
> *suffix)
>          }
>      }
>  }
> +
> +/* Serialized as: (device name\0 + lchs struct) x devices */

Comment is outdated.

> +    if (!mc->legacy_fw_cfg_order) {
> +        buf = get_boot_devices_lchs_list(&len);
> +        ptr = fw_cfg_modify_file(s, "bios-geometry", (uint8_t *)buf, len);

Can fw_cfg_modify_file handle buf == NULL?

cheers,
  Gerd