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

[Sean Christopherson <seanjc@google.com>]

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?)
> 
> Anyway, if this is an issue, it's orthogonal to this patch.

Pawan, any idea?