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

[Peter Zijlstra]

On Thu, Aug 01, 2019 at 06:57:41PM +0000, Lendacky, Thomas wrote:
> From: Tom Lendacky <thomas.lendacky@xxxxxxx>
> 
> It turns out that the NMI latency workaround from commit 6d3edaae16c6
> ("x86/perf/amd: Resolve NMI latency issues for active PMCs") ends up
> being too conservative and results in the perf NMI handler claiming NMIs
> to easily on AMD hardware when the NMI watchdog is active.
> 
> This has an impact, for example, on the hpwdt (HPE watchdog timer) module.
> This module can produce an NMI that is used to reset the system. It
> registers an NMI handler for the NMI_UNKNOWN type and relies on the fact
> that nothing has claimed an NMI so that its handler will be invoked when
> the watchdog device produces an NMI. After the referenced commit, the
> hpwdt module is unable to process its generated NMI if the NMI watchdog is
> active, because the current NMI latency mitigation results in the NMI
> being claimed by the perf NMI handler.
> 
> Update the AMD perf NMI latency mitigation workaround to, instead, use a
> window of time. Whenever a PMC is handled in the perf NMI handler, set a
> timestamp which will act as a perf NMI window. Any NMIs arriving within
> that window will be claimed by perf. Anything outside that window will
> not be claimed by perf. The value for the NMI window is set to 100 msecs.
> This is a conservative value that easily covers any NMI latency in the
> hardware. While this still results in a window in which the hpwdt module
> will not receive its NMI, the window is now much, much smaller.

Blergh, I so hate all this. The proposed patch is basically duct tape.

The horribly retarded x86 NMI infrastructure strikes again :/

Tom; do you have any idea how expensive it is to twiddle CR8 and play
games with interrupt priorities instead of piling world + dog on this
one NMI line? (as compared to CLI/STI)