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: <aD43icQolCvESIpc@google.com>
Date: Mon, 2 Jun 2025 16:45:13 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.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, Pawan Gupta wrote:
> 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.

Sounds good.

Ah, and the s/mmio_stale_data_clear/cpu_buf_vm_clear rename already landed for
6.16-rc1, so we don't have to overthink about the ordering with respect to that
change. :-)