From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 3/5] x86/kvm: Convert some slow-path static_cpu_has()
 callers to boot_cpu_has()
Date: Mon, 1 Apr 2019 09:24:06 +0200
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Cc: LKML <linux-kernel@vger.kernel.org>,
        Nadav Amit <nadav.amit@gmail.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Peter Zijlstra <peterz@infradead.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, x86@kernel.org
To: Borislav Petkov <bp@alien8.de>
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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
>