linux-kernel - Re: [PATCH 3/3] x86/xen: Move paravirt IOPL switching to slow the path

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Date:   Wed, 14 Jun 2017 10:40:17 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Brian Gerst <brgerst@...il.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH 3/3] x86/xen: Move paravirt IOPL switching to slow the path

On Wed, Jun 14, 2017 at 5:40 AM, Brian Gerst <brgerst@...il.com> wrote:
> Since tasks using IOPL are very rare, move the switching code to the slow
> path for lower impact on normal tasks.

I think that Andrew Cooper added a vmassist that we could opt in to
that makes Xen PV IOPL switching work more or less just like native.
We could maybe opt in to that and avoid needing this stuff at all on
newer hypervisors.