Re: [Qemu-devel] [PATCH 2/5] exec: qemu_ram_alloc_device, qemu_ram_resize

[Paolo Bonzini]

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