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

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Message-ID: <e8488e5c-7372-fc6e-daee-56633028854a@redhat.com>
Date:   Sun, 27 Mar 2022 12:43:36 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jon Kohler <jon@...anix.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     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 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.

>> Not that it really matters, since static_cpu_has() will patch out all the branches,
>> and in practice who cares about a JMP or NOP(s)?  But...
> 
> The reason I’ve been pursuing this is that the guest+host xsave adds up to
> a bit over ~1% as measured by perf top in an exit heavy workload. This is
> the first in a few patch we’ve drummed up to to get it back towards zero.
> I’ll send the rest out next week.

Can you add a testcase to x86/vmexit.c in kvm-unit-tests, too?

Thanks,

Paolo