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

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Message-ID: <20231201200247.vui6enzdj5nzctf4@treble>
Date:   Fri, 1 Dec 2023 12:02:47 -0800
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.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>,
        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, 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().

-- 
Josh