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Date:   Mon, 12 Feb 2018 12:50:02 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ingo Molnar <mingo@kernel.org>
Cc:     hpa@zytor.com, tglx@linutronix.de, torvalds@linux-foundation.org,
        linux-kernel@vger.kernel.org, dwmw@amazon.co.uk,
        linux-tip-commits@vger.kernel.org, Dave Hansen <dave@sr71.net>,
        Borislav Petkov <bp@alien8.de>,
        Arjan van de Ven <arjan@infradead.org>
Subject: Re: [tip:x86/pti] x86/speculation: Use IBRS if available before
 calling into firmware
Message-ID: <20180212115002.GI25181@hirez.programming.kicks-ass.net>
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 <20180212102211.cdrrqqd4hdw7xu5y@gmail.com>
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On Mon, Feb 12, 2018 at 11:22:11AM +0100, Ingo Molnar wrote:
> > +static inline void firmware_restrict_branch_speculation_start(void)
> > +{
> > +	alternative_msr_write(MSR_IA32_SPEC_CTRL, SPEC_CTRL_IBRS,
> > +			      X86_FEATURE_USE_IBRS_FW);
> > +}
> > +
> > +static inline void firmware_restrict_branch_speculation_end(void)
> > +{
> > +	alternative_msr_write(MSR_IA32_SPEC_CTRL, 0,
> > +			      X86_FEATURE_USE_IBRS_FW);
> 
> BTW., there's a detail that only occurred to me today, this enabling/disabling 
> sequence is not NMI safe, and it might be called from NMI context:

Wait, we're doing firmware from NMI? That sounds like a _REALLY_ bad
idea.