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: <CALMp9eRpP0LvMJ=aYf45xxz1fRrx5Sf9ZrqRE8yKRcMX-+f4+A@mail.gmail.com>
Date: Mon, 27 Oct 2025 15:03:23 -0700
From: Jim Mattson <jmattson@...gle.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: Sean Christopherson <seanjc@...gle.com>, 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 6:20 PM Pawan Gupta
<pawan.kumar.gupta@...ux.intel.com> wrote:
>
> ...
> 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 :-)

KVM started reporting MD_CLEAR to userspace in Linux v5.2, but it
didn't report L1D_FLUSH to userspace until Linux v6.4, so there are
plenty of virtual CPUs with a flushing VERW that don't have the L1D
flushing feature.