Re: [PATCH 1/3 V7] KVM, SEV: Add support for SEV intra host migration

[Peter Gonda]

On Thu, Sep 9, 2021 at 7:12 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Fri, Sep 10, 2021, Sean Christopherson wrote:
> > Ooh, this brings up a potential shortcoming of requiring @dst to be SEV-enabled.
> > If every SEV{-ES} ASID is allocated, then there won't be an available ASID to
> > (temporarily) allocate for the intra-host migration.  But that temporary ASID
> > isn't actually necessary, i.e. there's no reason intra-host migration should fail
> > if all ASIDs are in-use.

Ack forcing dst to be SEV disabled will mitigate this problem.

>
> ...
>
> > So I think the only option is to take vcpu->mutex for all vCPUs in both @src and
> > @dst.  Adding that after acquiring kvm->lock in svm_sev_lock_for_migration()
> > should Just Work.  Unless userspace is misbehaving, the lock won't be contended
> > since all vCPUs need to be quiesced, though it's probably worth using the
> > mutex_lock_killable() variant just to be safe.
>
> Circling back to this after looking at the SEV-ES support, I think the vCPUs in
> the source VM need to be reset via kvm_vcpu_reset(vcpu, false).  I doubt there's
> a use case for actually doing anything with the vCPU, but leaving it runnable
> without purging state makes me nervous.
>
> Alternative #1 would be to mark vCPUs as dead in some way so as to prevent doing
> anything useful with the vCPU.
>
> Alternative #2 would be to "kill" the source VM by setting kvm->vm_bugged to
> prevent all ioctls().
>
> The downside to preventing future ioctls() is that this would need to be the
> very last step of migration.  Not sure if that's problematic?

I'll add calls to kvm_vcpu_reset. Alternative #2 using vm_bugged won't
work for us because we need to keep using the source VM even after the
state is transfered.