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Date:   Mon, 12 Feb 2018 17:58:35 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Dave Hansen <dave@sr71.net>
Cc:     Ingo Molnar <mingo@kernel.org>, 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,
        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: <20180212165835.GO25181@hirez.programming.kicks-ass.net>
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On Mon, Feb 12, 2018 at 08:13:31AM -0800, Dave Hansen wrote:
> On 02/12/2018 02:22 AM, Ingo Molnar wrote:
> >> +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:
> 
> FWIW, Tim Chen and I talked about this a bunch.  We ended up just
> saving/restoring the MSR verbatim in the NMI handler the same way we do
> CR3, stashing it in a high general-purpose-register (r%12?).  That costs
> a RDMSR (at least) and an WRMSR (which you can optimize out).  We have a
> patch for that somewhere if anybody wants it.

I would really rather not do that on the NMI path.. And if we _have_ to,
please keep a software shadow of that MSR state, such that we can avoid
touching that MSR 99% of the time.