Re: [PATCH] pci: make ROM memory resizable

[Igor Mammedov]

On Tue, 25 Apr 2023 13:55:55 +0300
Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> wrote:

> On 24.04.23 23:36, Vladimir Sementsov-Ogievskiy wrote:
> > On migration, on target we load local ROM file. But actual ROM content
> > migrates through migration channel. Original ROM content from local
> > file doesn't matter. But when size mismatch - we have an error like
> > 
> >   Size mismatch: 0000:00:03.0/virtio-net-pci.rom: 0x40000 != 0x80000: 
> > Invalid argument
> > 
> > Let's just allow resizing of ROM memory. This way migration is not
> > relate on local ROM file on target node which is loaded by default but
> > is not actually needed.
> > 
> > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>  
> 
> Seems, automatically reconfigure the memory on incoming migration is not so 
> easy.. ROM size really influence config of the device, and after some digging 
> in the code I still cannot say where it happens.
You can't change PCI device bars on migration as it practically
will mess up memory mapping (if you manage to do so), but
guest will still think that it has the old one. So you'd need
a guest side that will notice migration and deal with remapping.

(one way to work around it could be unplug nic, and once it's gone
migrate/replug on target side)

> 
> So, I'm going to suggest another way, new series "[PATCH 0/3] ROM migration" 
> will substitute this patch.
>