linux-kernel - Re: [PATCH] perf: Change PMCR write to read-modify-write

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Message-ID: <20190401081757.lbfsuoocltq4lysj@blommer>
Date:   Mon, 1 Apr 2019 09:17:58 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Sodagudi Prasad <psodagud@...eaurora.org>
Cc:     Julien Thierry <julien.thierry@....com>, will.deacon@....com,
        mingo@...hat.com, catalin.marinas@....com,
        linux-arm-kernel@...ts.infradead.org, peterz@...radead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: Change PMCR write to read-modify-write

On Thu, Mar 21, 2019 at 01:02:27PM -0700, Sodagudi Prasad wrote:
> On 2019-03-21 06:34, Julien Thierry wrote:
> > Hi Prasad,
> > 
> > On 21/03/2019 02:07, Prasad Sodagudi wrote:
> > > Preserves the bitfields of PMCR_EL0(AArch64) during PMU reset.
> > > Reset routine should write a 1 to PMCR.C and PMCR.P fields only
> > > to reset the counters. Other fields should not be changed
> > > as they could be set before PMU initialization and their
> > > value must be preserved even after reset.
> > 
> > Are there any particular bit you are concerned about? Apart from the RO
> > ones and the Res0 ones (to which we are already writing 0), I see:
> > 
> > DP -> irrelevant for non-secure
> > X -> This one is the only potentially interesting, however it resets to
> > an architecturally unknown value, so unless we know for a fact it was
> > set before hand, we probably want to clear it
> > D -> ignored when we have LC set (and we do)
> > E -> Since this is the function we use to reset the PMU on the current
> > CPU, we probably want to set this bit to 0 regardless of its previous
> > value
> > 
> > So, is there any issue this patch is solving?
> 
> Hi Julien,
> 
> Thanks for taking a look into this patch. Yes. On our Qualcomm platforms,
> observed that X bit is getting cleared by kernel.
> This bit is getting set by firmware for Qualcomm use cases and non-secure
> world is resetting without this patch.

Given the kernel reconfigures the PMU dynamically at runtime (e.g. to multiplex
events over counters), I don't follow how this is useful. Could you elaborate
on how specifically you intend to use this?

> I think, changing that register this register modifications to
> read-modify-write style make sense.

I disagree. 

In general, RESx bits may be allocated new meanings in future (and will reset
to UNKNOWN values), so it is not safe to simply preserve the reset value. Given
that, any kernel _must_ explicitly reset registers in order to ensure the
expected behaviour.

Thanks
Mark.