Re: [PATCH v2 3/7] arch/x86/amd/ibs: Fix re-arming IBS Fetch

[peterz@infradead.org]

On Tue, Sep 08, 2020 at 04:47:36PM -0500, Kim Phillips wrote:
> Stephane Eranian found a bug in that IBS' current Fetch counter was not
> being reset when the driver would write the new value to clear it along
> with the enable bit set, and found that adding an MSR write that would
> first disable IBS Fetch would make IBS Fetch reset its current count.
> 
> Indeed, the PPR for AMD Family 17h Model 31h B0 55803 Rev 0.54 - Sep 12,
> 2019 states "The periodic fetch counter is set to IbsFetchCnt [...] when
> IbsFetchEn is changed from 0 to 1."
> 
> Explicitly set IbsFetchEn to 0 and then to 1 when re-enabling IBS Fetch,
> so the driver properly resets the internal counter to 0 and IBS
> Fetch starts counting again.
> 
> A family 15h machine tested does not have this problem, and the extra
> wrmsr is also not needed on Family 19h, so only do the extra wrmsr on
> families 16h through 18h.

*groan*...

> ---
>  arch/x86/events/amd/ibs.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
> index 26c36357c4c9..3eb9a55e998c 100644
> --- a/arch/x86/events/amd/ibs.c
> +++ b/arch/x86/events/amd/ibs.c
> @@ -363,7 +363,14 @@ perf_ibs_event_update(struct perf_ibs *perf_ibs, struct perf_event *event,
>  static inline void perf_ibs_enable_event(struct perf_ibs *perf_ibs,
>  					 struct hw_perf_event *hwc, u64 config)
>  {
> -	wrmsrl(hwc->config_base, hwc->config | config | perf_ibs->enable_mask);
> +	u64 _config = (hwc->config | config) & ~perf_ibs->enable_mask;
> +
> +	/* On Fam17h, the periodic fetch counter is set when IbsFetchEn is changed from 0 to 1 */
> +	if (perf_ibs == &perf_ibs_fetch && boot_cpu_data.x86 >= 0x16 && boot_cpu_data.x86 <= 0x18)
> +		wrmsrl(hwc->config_base, _config);

That's an anti-patttern (and for some reason this is the second time in
a week I've seen it). This is a fairly hot path and you're adding a
whole bunch of loads and branches.

Granted, in case you need the extra wrmsr that's probably noise, but
supposedly you're going to be fixing this in hardware eventually, and
you'll be getting rid of the second wrmsr again. But then you're stuck
with the loads and branches.

A better option would be to use hwc->flags, you're loading from that
line already, so it's guaranteed hot and then you only have a single
branch. Or stick it in perf_ibs near enable_mask, same difference.

> + 	_config |= perf_ibs->enable_mask;
> +	wrmsrl(hwc->config_base, _config);
>  }
>  
>  /*
> -- 
> 2.27.0
>