linux-kernel - Re: [PATCH 1/5] x86/ibrs: Introduce native_rdmsrl, and native

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Date:   Fri, 12 Jan 2018 16:28:45 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Dave Hansen <dave.hansen@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Andy Lutomirski <luto@...nel.org>
CC:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Andrea Arcangeli" <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        "Arjan Van De Ven" <arjan.van.de.ven@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>
Subject: Re: [PATCH 1/5] x86/ibrs: Introduce native_rdmsrl, and
 native_wrmsrl

On Thu, 2018-01-11 at 21:03 -0800, Dave Hansen wrote:
> On 01/11/2018 07:01 PM, Raj, Ashok wrote:
> > On Thu, Jan 11, 2018 at 06:20:13PM -0800, Andy Lutomirski wrote:
> >> On Thu, Jan 11, 2018 at 5:52 PM, Raj, Ashok <ashok.raj@...el.com> wrote:
> >>>>
> >>>> What's wrong with native_read_msr()?
> >>>
> >>> Yes, i think i should have added to msr.h. The names didn't read as a
> >>> pair, one was native_read_msr, wrmsrl could be taken over when paravirt is
> >>> defined?
> >>
> >> Why do you need to override paravirt?
> > 
> > The idea was since these MSR's are passed through we shouldn't need to 
> > handle them any differently. Also its best to do this as soon as possible
> > and avoid longer paths to get this barrier to hardware.
> 
> We were also worried about the indirect calls that are part of the
> paravirt interfaces when retpolines are not in place.

I have repeatedly been told that these go away, and are converted to
direct branches by the time the kernel is running userspace.

I suppose I really ought to validate that with qemu -d in_asm or
something. I couldn't see it in the code... but I haven't looked that
hard yet.
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