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

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Date:   Tue, 4 May 2021 14:13:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Suthikulpanit, Suravee" <suravee.suthikulpanit@....com>
Cc:     linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        iommu@...ts.linux-foundation.org, mingo@...hat.com,
        joro@...tes.org, Jon.Grimm@....com, amonakov@...ras.ru,
        David Coe <david.coe@...e.co.uk>
Subject: Re: [PATCH] x86/events/amd/iommu: Fix invalid Perf result due to
 IOMMU PMC power-gating

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?