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

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Message-ID: <20230601042345.52s5337uz62p6aow@treble>
Date:   Wed, 31 May 2023 21:23:45 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc:     Andrew Cooper <andrew.cooper3@...rix.com>,
        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 Wed, May 31, 2023 at 06:24:29PM -0700, Pawan Gupta wrote:

## 2023-05-31
> 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.

Yes.  Though in practice it might not make much of a difference.  With
wrmsr+lfence, the lfence has nothing to do so it might be almost
instantaneous anyway.

-- 
Josh