linux-kernel - Re: [PATCH 00/21] TDX/SNP part 1 of n, for 6.9

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Date: Wed, 28 Feb 2024 18:20:27 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, michael.roth@....com, 
	isaku.yamahata@...el.com, thomas.lendacky@....com
Subject: Re: [PATCH 00/21] TDX/SNP part 1 of n, for 6.9

On Wed, Feb 28, 2024 at 5:39 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > > This doesn't work.  The ENC flag gets set on any SNP *capable* CPU, which results
> > > in false positives for SEV and SEV-ES guests[*].
> >
> > You didn't look at the patch did you? :)
>
> Guilty, sort of.  I looked (and tested) the patch from the TDX series, but I didn't
> look at what you postd.  But it's a moot point, because now I did look at what you
> posted, and it's still broken :-)
>
> > It does check for has_private_mem (alternatively I could have dropped the bit
> > in SVM code for SEV and SEV-ES guests).
>
> The problem isn't with *KVM* setting the bit, it's with *hardware* setting the
> bit for SEV and SEV-ES guests.  That results in this:
>
>   .is_private = vcpu->kvm->arch.has_private_mem && (err & PFERR_GUEST_ENC_MASK),
>
> marking the fault as private.  Which, in a vacuum, isn't technically wrong, since
> from hardware's perspective the vCPU access was "private".  But from KVM's
> perspective, SEV and SEV-ES guests don't have private memory

vcpu->kvm->arch.has_private_mem is the flag from the SEV VM types
series. It's false on SEV and SEV-ES VMs, therefore fault->is_private
is going to be false as well. Is it ENOCOFFEE for you or ENODINNER for
me? :)

Paolo

> And because the flag only gets set on SNP capable hardware (in my limited testing
> of a whole two systems), running the same VM on different hardware would result
> in faults being marked private on one system, but not the other.  Which means that
> KVM can't rely on the flag being set for SEV or SEV-ES guests, i.e. we can't
> retroactively enforce anything (not to mention that that might break existing VMs).
>