linux-kernel - Re: [PATCH 3/5] x86/kvm: Convert some slow-path static_cpu_has() callers to boot_cpu

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Message-ID: <86dc0d5c-88af-dddc-4618-1a0a5a545e5f@redhat.com>
Date:   Mon, 1 Apr 2019 09:24:06 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Peter Zijlstra <peterz@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org
Subject: Re: [PATCH 3/5] x86/kvm: Convert some slow-path static_cpu_has()
 callers to boot_cpu_has()

On 31/03/19 17:12, Borislav Petkov wrote:
> On Sun, Mar 31, 2019 at 04:20:11PM +0200, Paolo Bonzini wrote:
>> These are not slow path.
> 
> Those functions do a *lot* of stuff like a bunch of MSR reads which are
> tens of cycles each at least.

The MSR reads and writes are not done in the common case.  Also, you
cannot really expect boot_cpu_data to be in L1 in these functions since
they run after the guest---or if they do, each L1 line you fill in with
host data is one line you "steal" from the guest.

Paolo

> I don't think a RIP-relative MOV and a BT:
> 
>         movq    boot_cpu_data+20(%rip), %rax    # MEM[(const long unsigned int *)&boot_cpu_data + 20B], _45
>         btq     $59, %rax       #, _45
> 
> are at all noticeable.
> 
> On latest AMD and Intel uarch those are 2-4 cycles, according to
> 
> https://agner.org/optimize/instruction_tables.ods
>