linux-kernel - Re: [PATCH] KVM: VMX: remove LFENCE in vmx_spec_ctrl_restore

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Message-ID: <20230601012323.36te7hfv366danpf@desk>
Date:   Wed, 31 May 2023 18:24:29 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Josh Poimboeuf <jpoimboe@...nel.org>, Jon Kohler <jon@...anix.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        "kvm @ vger . kernel . org" <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: VMX: remove LFENCE in vmx_spec_ctrl_restore_host()

On Thu, Jun 01, 2023 at 01:50:48AM +0100, Andrew Cooper wrote:
> On 01/06/2023 1:42 am, Josh Poimboeuf wrote:
> > So each LFENCE has a distinct purpose.  That said, there are no indirect
> > branches or unbalanced RETs between them.
> 
> How lucky are you feeling?
> 
> You're in C at this point, which means the compiler could have emitted a
> call to mem{cpy,cmp}() in place of a simple assignment/comparison.

Moving the second LFENCE to the else part of WRMSR should be possible?
So that the serialization can be achived either by WRMSR or LFENCE. This
saves an LFENCE when host and guest value of MSR_SPEC_CTRL differ.

---
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index d2d6e1b6c788..d32e6d172dd6 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7157,8 +7157,8 @@ void noinstr vmx_spec_ctrl_restore_host(struct vcpu_vmx *vmx,
 	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) ||
 	    vmx->spec_ctrl != hostval)
 		native_wrmsrl(MSR_IA32_SPEC_CTRL, hostval);
-
-	barrier_nospec();
+	else
+		barrier_nospec();
 }
 
 static fastpath_t vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu)