linux-kernel - Re: [PATCH 3/5] KVM: VMX: Apply MMIO Stale Data mitigation if KVM maps MMIO into the guest

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Message-ID: <20250529234013.fbxruxq44wpfh5w4@desk>
Date: Thu, 29 May 2025 16:40:13 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
	Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH 3/5] KVM: VMX: Apply MMIO Stale Data mitigation if KVM
 maps MMIO into the guest

On Thu, May 29, 2025 at 03:19:22PM -0700, Sean Christopherson wrote:
> On Wed, May 28, 2025, Pawan Gupta wrote:
> > On Thu, May 22, 2025 at 06:17:54PM -0700, Sean Christopherson wrote:
> > > @@ -7282,7 +7288,7 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
> > >  	if (static_branch_unlikely(&vmx_l1d_should_flush))
> > >  		vmx_l1d_flush(vcpu);
> > >  	else if (static_branch_unlikely(&mmio_stale_data_clear) &&
> > > -		 kvm_arch_has_assigned_device(vcpu->kvm))
> > > +		 (flags & VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO))
> > >  		mds_clear_cpu_buffers();
> > 
> > I think this also paves way for buffer clear for MDS and MMIO to be done at
> > a single place. Please let me know if below is feasible:
> 
> It's definitely feasible (this thought crossed my mind as well), but because
> CLEAR_CPU_BUFFERS emits VERW iff X86_FEATURE_CLEAR_CPU_BUF is enabled, the below
> would do nothing for the MMIO case (either that, or I'm missing something).

Thats right, CLEAR_CPU_BUFFERS needs rework too.

> We could obviously rework CLEAR_CPU_BUFFERS, I'm just not sure that's worth the
> effort at this point.  I'm definitely not opposed to it though.

My goal with this is to have 2 separate controls for user-kernel and
guest-host. Such that MDS/TAA/RFDS gets finer controls to only enable
user-kernel or guest-host mitigation. This would play well with the Attack
vector series by David:

https://lore.kernel.org/lkml/20250509162839.3057217-1-david.kaplan@amd.com/

For now this patch is fine as is. I will send update separately including
the CLEAR_CPU_BUFFERS rework.