Re: [PATCH v12 08/28] target/i386: add Secure Encrypted Virtulization (SEV) object

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Eduardo Habkost (ehabkost@redhat.com) wrote:
> On Tue, Mar 13, 2018 at 08:04:51PM +0100, Paolo Bonzini wrote:
> > On 13/03/2018 19:49, Eduardo Habkost wrote:
> > >>>
> > >>> Exactly, in other words these two options are part of the guest
> > >>> ABI, and QEMU promises to never make the guest ABI depend on the
> > >>> host hardware unless you're using "-cpu host".
> > >>
> > >> This is not entirely true; while MAXPHYADDR is constant downstream
> > >> unless using "-cpu host", in practice that behavior is wrong and a guest
> > >> could misbehave if passed a MAXPHYADDR that is different from the host's.
> > >>
> > >> I think this is the same, and management software will have to live with it.
> > > 
> > > I think they are very far from being equivalent.
> > 
> > Right, I only meant to say that guest ABI actually does depend on the
> > host hardware, even outside of "-cpu host".
> > 
> > > But if you tell the guest the wrong C-bit location, guests are
> > > likely to rely on it and break.  Migration between hosts with
> > > different C-bit locations won't work, will it?
> > 
> > It won't---but as long as the destination hosts fails fast when the
> > C-bit location is wrong, it's okay.  What matters is that we don't run
> > guest code with the wrong C bit, as you noted.
> 
> Are you proposing we change the default to simply use cbitpos
> from the host?

Hmm I don't like that idea; as an option that's fine, as the only
way it's not.

> I would agree with this only if we make QEMU able to prevent live
> migration to a host with mismatching cbitpos.

Yeh; especially since I suspect debugging stuff with a failed SEV
migration like that is going to be really hard.

Dave

> -- 
> Eduardo
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK