linux-kernel - Re: [PATCH] perf/x86/amd: Change NMI latency mitigation to use a timestamp

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Message-ID: <20190913195255.GA20131@anatevka.americas.hpqcorp.net>
Date:   Fri, 13 Sep 2019 13:52:55 -0600
From:   Jerry Hoemann <jerry.hoemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>
Subject: Re: [PATCH] perf/x86/amd: Change NMI latency mitigation to use a
 timestamp

On Thu, Aug 08, 2019 at 02:33:24PM -0600, Jerry Hoemann wrote:
> On Fri, Aug 02, 2019 at 06:33:28PM +0200, Peter Zijlstra wrote:
> > On Fri, Aug 02, 2019 at 06:20:15PM +0200, Peter Zijlstra wrote:
> > > On Fri, Aug 02, 2019 at 02:33:41PM +0000, Lendacky, Thomas wrote:
> > 
> > > > Talking to the hardware folks, they say setting CR8 is a serializing
> > > > instruction and has to communicate out to the APIC, so it's better to
> > > > use CLI/STI.
> > > 
> > > Bah; the Intel SDM states: "MOV CR* instructions, except for MOV CR8,
> > > are serializing instructions", which had given me a little hope.
> > > 
> > > At the same time, all these chips still have the APIC TPR field too, so
> > > much like how the TSC DEADLINE MSR is a hidden APIC write, so too is CR8
> > > I suppose :-(
> > > 
> > > I'll still finish the patches I started, just to see what it would look
> > > like.
> > 
> > Another 'fun' issue I ran into while doing these patches; STI has a 1
> > instruction shadow, which we rely on, MOV CR8 does not. So things like:
> > 
> > native_safe_halt:
> > 	sti
> > 	hlt
> > 
> > turn into:
> > 
> > native_safe_halt:
> > 	cli
> > 	movl $0, %rax
> > 	movq %rax, %cr8
> > 	sti
> > 	hlt
> > 
> 
> Hi Peter,
> 
> What is our the next step here?
> 
> Are you still looking to make this change?
> 
> Do we want to pick up Tom Lendacky's patch on an interim basis while
> you're working on the bigger change?  (I can say we tested Tom's
> patch and it does address the issue we were seeing.)
> 
> Thanks
> 
> Jerry
> 

Hi Peter,

Have you gotten a chance to look into this more?

Thanks

Jerry


-- 

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Jerry Hoemann                  Software Engineer   Hewlett Packard Enterprise
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