linux-kernel - Re: [PATCH v4 6/6] KVM: VMX: Move VERW closer to VMentry for MDS mitigation

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Message-ID: <20231220012557.7myc7xy24tveaid6@desk>
Date: Tue, 19 Dec 2023 17:25:57 -0800
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>
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>,
	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,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Nikolay Borisov <nik.borisov@...e.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,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH  v4 6/6] KVM: VMX: Move VERW closer to VMentry for MDS
 mitigation

On Fri, Dec 01, 2023 at 12:02:47PM -0800, Josh Poimboeuf wrote:
> On Fri, Oct 27, 2023 at 07:39:12AM -0700, Pawan Gupta wrote:
> > -	vmx_disable_fb_clear(vmx);
> > +	/*
> > +	 * Optimize the latency of VERW in guests for MMIO mitigation. Skip
> > +	 * the optimization when MDS mitigation(later in asm) is enabled.
> > +	 */
> > +	if (!cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF))
> > +		vmx_disable_fb_clear(vmx);
> >  
> >  	if (vcpu->arch.cr2 != native_read_cr2())
> >  		native_write_cr2(vcpu->arch.cr2);
> > @@ -7248,7 +7256,8 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
> >  
> >  	vmx->idt_vectoring_info = 0;
> >  
> > -	vmx_enable_fb_clear(vmx);
> > +	if (!cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF))
> > +		vmx_enable_fb_clear(vmx);
> >  
> 
> It may be cleaner to instead check X86_FEATURE_CLEAR_CPU_BUF when
> setting vmx->disable_fb_clear in the first place, in
> vmx_update_fb_clear_dis().

Right. Thanks for the review.