linux-kernel - Re: [PATCH] KVM: x86: optimize PKU branching in kvm_load_{guest|host}_xsave

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Date:   Mon, 28 Mar 2022 14:51:21 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jon Kohler <jon@...anix.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        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" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: optimize PKU branching in
 kvm_load_{guest|host}_xsave_state

On Mon, Mar 28, 2022, Jon Kohler wrote:
> 
> 
> > On Mar 27, 2022, at 6:43 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> > 
> > On 3/26/22 02:37, Jon Kohler wrote:
> >>>>    Flip the ordering of the || condition so that XFEATURE_MASK_PKRU is
> >>>>    checked first, which when instrumented in our environment appeared
> >>>>    to be always true and less overall work than kvm_read_cr4_bits.
> >>> 
> >>> If it's always true, then it should be checked last, not first.  And if
> >> Sean thanks for the review. This would be a left handed || short circuit, so
> >> wouldn’t we want always true to be first?
> > 
> > Yes.
> 
> Ack, thanks.

Yeah, I lost track of whether it was a || or &&.