linux-kernel - Re: [PATCH v2] x86/ibpb: Skip IBPB when we switch back to same user process

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Message-ID: <1517135905.6624.68.camel@amazon.co.uk>
Date:   Sun, 28 Jan 2018 10:38:25 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Ingo Molnar <mingo@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
CC:     <linux-kernel@...r.kernel.org>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Andy Lutomirski" <luto@...nel.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        "Ashok Raj" <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        "Borislav Petkov" <bp@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@....com>,
        Joerg Roedel <joro@...tes.org>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Laura Abbott <labbott@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tom Lendacky <thomas.lendacky@....com>, <x86@...nel.org>
Subject: Re: [PATCH v2] x86/ibpb: Skip IBPB when we switch back to same
 user process

On Sun, 2018-01-28 at 10:56 +0100, Ingo Molnar wrote:
> 
> What tree is this patch against? It doesn't apply to linus's latest, nor to 
> tip:master.

It's in my tree at
 http://git.infradead.org/users/dwmw2/linux-retpoline.git/shortlog/refs/heads/ibpb
which is gradually being finalized and flushed via tip/x86/pti.

The IBPB on context switch parts are currently the first three patches
there, which roughly suggests they might be the next to get sent out
for real, if we have reached a consensus. The three patches there
probably want collapsing into one, but I've left them as-is for now
while we're discussing it.

The other thing that's next on the list is exposing the MSRs to guests.
The IBPB one is fairly simple, and Karim is working on exposing IBRS to
guests too, using Paolo's per-vCPU MSR bitmap to do the same trick
we've done in Xen, to expose it only after the guest first touches it
(to avoid the cost of swapping it when it's always 0←→0 in the common
case). I think Ashok was talking about doing the same thing? We'll see
who gets there first :)
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