linux-kernel - Re: [PATCH v3 1/4] KVM: VMX: Flush CPU buffers as needed if L1D cache flush is skipped

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Message-ID: <20251022012021.sbymuvzzvx4qeztf@desk>
Date: Tue, 21 Oct 2025 18:20:21 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Brendan Jackman <jackmanb@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/4] KVM: VMX: Flush CPU buffers as needed if L1D
 cache flush is skipped

On Tue, Oct 21, 2025 at 04:30:19PM -0700, Pawan Gupta wrote:
> On Tue, Oct 21, 2025 at 09:48:30AM -0700, Sean Christopherson wrote:
> > On Tue, Oct 21, 2025, Brendan Jackman wrote:
> > > On Thu Oct 16, 2025 at 8:04 PM UTC, Sean Christopherson wrote:
> > > > If the L1D flush for L1TF is conditionally enabled, flush CPU buffers to
> > > > mitigate MMIO Stale Data as needed if KVM skips the L1D flush, e.g.
> > > > because none of the "heavy" paths that trigger an L1D flush were tripped
> > > > since the last VM-Enter.
> > > 
> > > Presumably the assumption here was that the L1TF conditionality is good
> > > enough for the MMIO stale data vuln too? I'm not qualified to assess if
> > > that assumption is true, but also even if it's a good one it's
> > > definitely not obvious to users that the mitigation you pick for L1TF
> > > has this side-effect. So I think I'm on board with calling this a bug.
> > 
> > Yeah, that's where I'm at as well.
> > 
> > > If anyone turns out to be depending on the current behaviour for
> > > performance I think they should probably add it back as a separate flag.
> > 
> > ...
> > 
> > > > @@ -6722,6 +6722,7 @@ static noinstr void vmx_l1d_flush(struct kvm_vcpu *vcpu)
> > > >  		:: [flush_pages] "r" (vmx_l1d_flush_pages),
> > > >  		    [size] "r" (size)
> > > >  		: "eax", "ebx", "ecx", "edx");
> > > > +	return true;
> > > 
> > > The comment in the caller says the L1D flush "includes CPU buffer clear
> > > to mitigate MDS" - do we actually know that this software sequence
> > > mitigates the MMIO stale data vuln like the verw does? (Do we even know if
> > > it mitigates MDS?)

Thinking more on this, the software sequence is only invoked when the
system doesn't have the L1D flushing feature added by a microcode update.
In such a case system is not expected to have a flushing VERW either, which
was introduced after L1TF. Also, the admin needs to have a very good reason
for not updating the microcode for 5+ years :-)

Anyways, I have asked for a confirmation if the sequence works for MMIO
stale data also. I will update once I get a response.

> > > Anyway, if this is an issue, it's orthogonal to this patch.
> > 
> > Pawan, any idea?
> 
> I want to say yes, but let me first confirm this internally and get back to
> you.