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