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

[Peter Zijlstra]

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