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From:   Jim Mattson <jmattson@google.com>
Date:   Wed, 14 Feb 2018 16:46:46 -0800
Message-ID: <CALMp9eRk8WvcALB4wHKo+P=dv2Xs9os6=15g_bL2Uy5+4uPTJA@mail.gmail.com>
Subject: Re: [PATCH v2 1/4] x86/speculation: Use IBRS if available before
 calling into firmware
To:     David Woodhouse <dwmw@amazon.co.uk>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        KarimAllah Ahmed <karahmed@amazon.de>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        kvm list <kvm@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        "Van De Ven, Arjan" <arjan.van.de.ven@intel.com>,
        Dave Hansen <dave.hansen@intel.com>, mingo@kernel.org
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On Wed, Feb 14, 2018 at 3:29 PM, David Woodhouse <dwmw@amazon.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.