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

[Ian Campbell]

On Thu, 2014-02-06 at 17:30 +0400, Michael Tokarev wrote:
> 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 :)

Xen only supports migration from version N to N+1, and since this issue
is fixed it should be fine.

I'm not sure why the BIOS size is relevant to migration anyway, Xen
doesn't load a new BIOS on the target host, the BIOS image is part of
the migrated RAM.

Ian.