linux-kernel - Re: [PATCH 04/11] x86/fpu: eager switch PKRU state

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Message-ID: <09127264-29ba-8037-38a4-a10acc6552ea@linux.intel.com>
Date:   Fri, 12 Oct 2018 12:44:39 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH 04/11] x86/fpu: eager switch PKRU state

On 10/12/2018 11:09 AM, Andy Lutomirski wrote:
> But maybe WRPKRU is more expensive than RDPKRU and a branch?

Yeah, it is more expensive.  It has a higher cycle cost and it's also
practically a (light) speculation barrier.