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

[Peter Zijlstra]

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?