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Date: Fri, 1 Dec 2023 12:02:47 -0800
From: Josh Poimboeuf <jpoimboe@kernel.org>
To: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
	"H. Peter Anvin" <hpa@zytor.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
	Sean Christopherson <seanjc@google.com>,
	Paolo Bonzini <pbonzini@redhat.com>, tony.luck@intel.com,
	ak@linux.intel.com, tim.c.chen@linux.intel.com,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Nikolay Borisov <nik.borisov@suse.com>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	kvm@vger.kernel.org,
	Alyssa Milburn <alyssa.milburn@linux.intel.com>,
	Daniel Sneddon <daniel.sneddon@linux.intel.com>,
	antonio.gomez.iglesias@linux.intel.com,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH  v4 6/6] KVM: VMX: Move VERW closer to VMentry for MDS
 mitigation
Message-ID: <20231201200247.vui6enzdj5nzctf4@treble>
References: <20231027-delay-verw-v4-0-9a3622d4bcf7@linux.intel.com>
 <20231027-delay-verw-v4-6-9a3622d4bcf7@linux.intel.com>
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In-Reply-To: <20231027-delay-verw-v4-6-9a3622d4bcf7@linux.intel.com>

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