Re: [Qemu-devel] [QEMU] [PATCH v2 0/8] Add Qemu to SeaBIOS LCHS interface

[Gerd Hoffmann <kraxel@redhat.com>]

On Wed, Jun 12, 2019 at 02:59:31PM +0300, Sam Eiderman wrote:
> v1:
> 
> 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 guess - SeaBIOS, for large enough disks - will
> use LBA translation, which will report 63 SPT instead.

--verbose please.

As far I know seabios switches to LBA mode when the disk is simply too
big for LCHS addressing.  So I fail to see which problem is solved by
this.  If your guest needs LCHS, why do you assign a disk which can't
be fully accessed using LCHS addressing?

> In addition we can not enforce SeaBIOS to rely on phyiscal geometries at
> all. A virtio-blk-pci virtual disk with 255 phyiscal heads can not
> report more than 16 physical heads when moved to an IDE controller, the
> ATA spec allows a maximum of 16 heads - this is an artifact of
> virtualization.

Well, not really.  Moving disks from one controller to another when the
OS depends on LHCS addressing never is a good idea.  That already caused
problems in the 90-ies, when moving scsi disks from one scsi host
adapter to another type, *way* before virtualization became a thing.

BTW:  One possible way to figure which LCHS layout a disk uses is to
check the MBR partition table.  With that we (a) don't need a new
interface between qemu and seabios and (b) it is not needed to manually
specify the geometry.

cheers,
  Gerd