linux-kernel - Re: [PATCH v2 1/4] x86/speculation: Use IBRS if available before calling into firmware

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Date:   Thu, 15 Feb 2018 09:21:56 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        kvm list <kvm@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
        Dave Hansen <dave.hansen@...el.com>, mingo@...nel.org
Subject: Re: [PATCH v2 1/4] x86/speculation: Use IBRS if available before
 calling into firmware

On Wed, 2018-02-14 at 16:46 -0800, Jim Mattson wrote:
> On Wed, Feb 14, 2018 at 3:29 PM, David Woodhouse <dwmw@...zon.co.uk> wrote:
> 
> > +#define alternative_msr_write(_msr, _val, _feature)            \
> > +       asm volatile(ALTERNATIVE("",                            \
> > +                                "movl %[msr], %%ecx\n\t"       \
> > +                                "movl %[val], %%eax\n\t"       \
> > +                                "movl $0, %%edx\n\t"           \
> > +                                "wrmsr",                       \
> > +                                _feature)                      \
> > +                    : : [msr] "i" (_msr), [val] "i" (_val)     \
> > +                    : "eax", "ecx", "edx", "memory")
> > +
> 
> It's not needed now, but this would be more generally useful if the
> high 32 bits of the MSR value could also be specified.

Yeah, if we ever do want to make it completely generic then we could
move it from nospec-branch.h to alternative.h and make it possible to
set the high word. But for now I'm inclined not to overengineer it.

The idea *was* that Boris was going to bikeshed it away into some other
form, but having just reverted two of those patches I am less inclined
to accept any more of that kind of change that doesn't actually fix a
real bug.

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