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: <20251027231721.irprdsyqd2klt4bf@desk>
Date: Mon, 27 Oct 2025 16:17:21 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Jim Mattson <jmattson@...gle.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 Mon, Oct 27, 2025 at 03:03:23PM -0700, Jim Mattson wrote:
> 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.

Shouldn't only the L0 hypervisor be doing the L1D_FLUSH?

kvm_get_arch_capabilities()
{
...
        /*
         * If we're doing cache flushes (either "always" or "cond")
         * we will do one whenever the guest does a vmlaunch/vmresume.
         * If an outer hypervisor is doing the cache flush for us
         * (ARCH_CAP_SKIP_VMENTRY_L1DFLUSH), we can safely pass that
         * capability to the guest too, and if EPT is disabled we're not
         * vulnerable.  Overall, only VMENTER_L1D_FLUSH_NEVER will
         * require a nested hypervisor to do a flush of its own.
         */
        if (l1tf_vmx_mitigation != VMENTER_L1D_FLUSH_NEVER)
                data |= ARCH_CAP_SKIP_VMENTRY_L1DFLUSH;