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

[Paolo Bonzini]

Il 06/02/2014 15:48, Ian Campbell ha scritto:
> > > > > 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.
> > >
> > > The memory layout is different with a larger bios as qemu makes the
> > > virtual flash as big as the bios image is.  The different memory layout
> > > in turn makes live migration fail.
> OK. Xen doesn't emulate flash so it doesn't have this issue.

It's problematic even without flash.  At least in Xen, all ROM memory 
blocks are migrated, not just the main RAM.  Migration of RAM/ROM block 
data is independent from where the RAM/ROM is mapped in guest physical 
address space.

In the guest, a 128k BIOS is at fffe0000-ffffffff.

A 256k BIOS is at fffc0000-ffffffff.

If the destination prepares the memory map for a 256k BIOS, and the 
source sends the data for a 128k BIOS, the 128k BIOS is erroneously 
placed at ffffc0000-fffdffff.  There is no reset vector fffffff0.  The 
guest works fine until you reboot it, then it breaks.

Paolo