Re: [Qemu-devel] 128Kb bios size and older machines?

[Michael Tokarev <mjt@tls.msk.ru>]

06.02.2014 17:15, Ian Campbell wrote:
> On Thu, 2014-02-06 at 17:08 +0400, Michael Tokarev wrote:
>> 06.02.2014 12:50, Gerd Hoffmann wrote:
[]
>>> As far I know xen has its own seabios builds anyway as they
>>> have to add hvmloader to the mix.  Thats why it has been
>>> turned off in the upstream blob builds, nobody will use these
>>> with xen anyway.
>>
>> Thank you Gerd for the answer.  Ian, can you give some light
>> here, what is still missing in seabios for xen?  Because,
>> well, ..
> 
> Nothing is missing in seabios.
> 
> The above patch is a fix to Xen not SeaBIOS, which lets it work
> correctly regardless of the size of the seabios binary.
> 
> There is no need to do anything other than apply that fix to the Xen
> packages (perhaps by upgrading to 4.3.1) AFAIK.

Okay.  That is good to know, because it started to become
quite confusing :)

Yes I understand the xen hvmloader change is necessary for
xen to support larger bios sizes, and new full-blown bios
is larger than 128Kb.

[]
>> (And yes I know about
>>   http://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=5f2875739beef3a75c7a7e8579b6cbcb464e61b3
>> which talks about >128kb bios size.  This is actually exactly
>> the same issue which I'm trying to address now, from a few PoVs:
>> qemu now builds/uses stripped-down bios for older (pre-2.0) machine
>> types, and that stripped-down version does not include Xen bits
>> in order to fit in 128Kb, because with xen it doesn't fit anymore.
>> Maybe we should start building special xen variant of seabios --
>> stripped down but WITH xen.  Oh well.)
> 
> Why is this stripped down SeaBIOS needed?

Because it fits into 128Kb, while complete build doesn't.
And when the bios size grows (provided that software -
xen and qemu - actually support this), it breaks guest
migration between old and new systems.

> Is there some reason the Xen build can't pickup the non-stripped down
> (post-2.0) SeaBIOS?

The question is exactly about old systems.  Qemu has an option,
-M foo.  Right now, if foo is less than 2.0 (the upcoming release),
it picks up the old small and now stripped-down verion of seabios.
For new versions - like -M pc-2.0, or by default - it uses the
new larger bios with all features.

But I've no idea if xen actually uses or cares about -M to start
with.  Qemu supports it for 2 things: migration, and keeping, say,
windows guests activated across version changes (with variable
success on both fronts :)

Thanks,

/mjt