Re: [PATCH] x86/events/amd/iommu: Fix invalid Perf result due to IOMMU PMC power-gating

[Peter Zijlstra]

On Wed, May 05, 2021 at 07:39:14PM +0700, Suthikulpanit, Suravee wrote:
> Peter,
> 
> On 5/4/2021 7:13 PM, Peter Zijlstra wrote:
> > On Tue, May 04, 2021 at 06:58:29PM +0700, Suthikulpanit, Suravee wrote:
> > > Peter,
> > > 
> > > On 5/4/2021 4:39 PM, Peter Zijlstra wrote:
> > > > On Tue, May 04, 2021 at 01:52:36AM -0500, Suravee Suthikulpanit wrote:
> > > > 
> > > > > 2. Since AMD IOMMU PMU does not support interrupt mode, the logic
> > > > >      can be simplified to always start counting with value zero,
> > > > >      and accumulate the counter value when stopping without the need
> > > > >      to keep track and reprogram the counter with the previously read
> > > > >      counter value.
> > > > 
> > > > This relies on the hardware counter being the full 64bit wide, is it?
> > > > 
> > > 
> > > The HW counter value is 48-bit. Not sure why it needs to be 64-bit?
> > > I might be missing some points here? Could you please describe?
> > 
> > How do you deal with the 48bit overflow if you don't use the interrupt?
> 
> The IOMMU Perf driver does not currently handle counter overflow since the overflow
> notification mechanism (i.e. IOMMU creates an EVENT_COUNTER_ZERO event in the IOMMU event log,
> and generate an IOMMU MSI interrupt to signal IOMMU driver to process the event.) is not
> currently supported. When counter overflows, the counter becomes zero, and Perf
> reports value zero for the event.
> 
> Alternatively, to detect overflow, we might start counting with value 1 so that
> we can detect overflow when the value becomes zero in which case the Perf driver
> could generate error message.

Urgh.. the intel uncore driver programs an hrtimer to periodically fold
deltas. That way the counter will never be short.