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Message-ID: <20231026190741.vwpyvp3nvyrlcmsp@desk>
Date: Thu, 26 Oct 2023 12:07:41 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Nikolay Borisov <nik.borisov@...e.com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
"H. Peter Anvin" <hpa@...or.com>,
Peter Zijlstra <peterz@...radead.org>,
Josh Poimboeuf <jpoimboe@...nel.org>,
Andy Lutomirski <luto@...nel.org>,
Jonathan Corbet <corbet@....net>,
Sean Christopherson <seanjc@...gle.com>,
Paolo Bonzini <pbonzini@...hat.com>, tony.luck@...el.com,
ak@...ux.intel.com, tim.c.chen@...ux.intel.com,
linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
kvm@...r.kernel.org,
Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
antonio.gomez.iglesias@...ux.intel.com
Subject: Re: [PATCH v3 6/6] KVM: VMX: Move VERW closer to VMentry for MDS
mitigation
On Thu, Oct 26, 2023 at 07:14:18PM +0300, Nikolay Borisov wrote:
> > if (static_branch_unlikely(&vmx_l1d_should_flush))
> > vmx_l1d_flush(vcpu);
> > - else if (cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF))
> > - mds_clear_cpu_buffers();
> > else if (static_branch_unlikely(&mmio_stale_data_clear) &&
> > kvm_arch_has_assigned_device(vcpu->kvm))
> > + /* MMIO mitigation is mutually exclusive with MDS mitigation later in asm */
>
> Mutually exclusive implies that you have one or the other but not both,
> whilst I think the right formulation here is redundant? Because if mmio is
> enabled mds_clear_cpu_buffers() will clear the buffers here and later
> they'll be cleared again, no ?
No, because when mmio_stale_data_clear is enabled,
X86_FEATURE_CLEAR_CPU_BUF will not be set because of how mitigation is
selected in mmio_select_mitigation():
mmio_select_mitigation()
{
...
/*
* Enable CPU buffer clear mitigation for host and VMM if also affected
* by MDS or TAA. Otherwise, enable mitigation for VMM only.
*/
if (boot_cpu_has_bug(X86_BUG_MDS) || (boot_cpu_has_bug(X86_BUG_TAA) &&
boot_cpu_has(X86_FEATURE_RTM)))
setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
else
static_branch_enable(&mmio_stale_data_clear);
> Alternatively you might augment this check to only execute iff
> X86_FEATURE_CLEAR_CPU_BUF is not set?
It already is like that due to the logic above. That is what the
comment:
/* MMIO mitigation is mutually exclusive with MDS mitigation later in asm */
... is trying to convey. Suggestions welcome to improve the comment.
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