linux-kernel - Re: [PATCH v3 10/15] KVM: x86: add fields to struct kvm

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Date: Thu, 14 Mar 2024 18:48:50 -0500
From: Michael Roth <michael.roth@....com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, <linux-kernel@...r.kernel.org>,
	<kvm@...r.kernel.org>, <aik@....com>, <pankaj.gupta@....com>
Subject: Re: [PATCH v3 10/15] KVM: x86: add fields to struct kvm_arch for
 CoCo features

On Thu, Mar 14, 2024 at 03:56:27PM -0700, Sean Christopherson wrote:
> On Thu, Mar 14, 2024, Michael Roth wrote:
> > On Wed, Mar 13, 2024 at 09:49:52PM -0500, Michael Roth wrote:
> > > I've been trying to get SNP running on top of these patches and hit and
> > > issue with these due to fpstate_set_confidential() being done during
> > > svm_vcpu_create(), so when QEMU tries to sync FPU state prior to calling
> > > SNP_LAUNCH_FINISH it errors out. I think the same would happen with
> > > SEV-ES as well.
> > > 
> > > Maybe fpstate_set_confidential() should be relocated to SEV_LAUNCH_FINISH
> > > site as part of these patches?
> > 
> > Talked to Tom a bit about this and that might not make much sense unless
> > we actually want to add some code to sync that FPU state into the VMSA
> > prior to encryption/measurement. Otherwise, it might as well be set to
> > confidential as soon as vCPU is created.
> > 
> > And if userspace wants to write FPU register state that will not actually
> > become part of the guest state, it probably does make sense to return an
> > error for new VM types and leave it to userspace to deal with
> > special-casing that vs. the other ioctls like SET_REGS/SREGS/etc.
> 
> Won't regs and sregs suffer the same fate?  That might not matter _today_ for
> "real" VMs, but it would be a blocking issue for selftests, which need to stuff
> state to jumpstart vCPUs.

SET_REGS/SREGS and the others only throw an error when
vcpu->arch.guest_state_protected gets set, which doesn't happen until
sev_launch_update_vmsa(). So in those cases userspace is still able to sync
additional/non-reset state prior initial launch. It's just XSAVE/XSAVE2 that
are a bit more restrictive because they check fpstate_is_confidential()
instead, which gets set during vCPU creation.

Somewhat related, but just noticed that KVM_SET_FPU also relies on
fpstate_is_confidential() but still silently returns 0 with this series.
Seems like it should be handled the same way as XSAVE/XSAVE2, whatever we
end up doing.

-Mike

> 
> And maybe someday real VMs will catch up to the times and stop starting at the
> RESET vector...
>