On 19/11/2014 17:27, Kevin O'Connor wrote: > On Wed, Nov 19, 2014 at 02:44:32PM +0100, Paolo Bonzini wrote: >> So: >> >> qemu-2.0 -M pc-2.0 -> qemu-2.2 -M pc-2.0 >> >> uses 2.0 BIOS >> >> qemu-2.2 -M pc-2.0 -> qemu-2.0 -M pc-2.0 >> >> uses 2.2 BIOS >> >> Both should work, in general. BIOS is rarely the reason for >> incompatibilities. However, breakage can happen, for example I know >> that RHEL7 SeaBIOS does not work on RHEL6. RHEL6 SeaBIOS works on >> RHEL7, but it needs a couple workarounds. > > Do you know why that is? We do try to make SeaBIOS backwards > compatible with older versions of QEMU.
For RHEL6-on-RHEL7 SeaBIOS was violating the virtio-scsi spec, but QEMU happened to support the broken case until recently. When I was told that the broken case stopped working, I added a workaround. I don't know why RHEL7 SeaBIOS does not work on RHEL6. But note that it's a really old version (0.12). I guess one could try compiling various QEMU versions and see where it broke, and then either debug or bisect further. RHEL7 uses 1.5.3. Paolo
