linux-kernel - Re: [PATCH] clocksource: Add Marvell Errata-38627 workaround

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Message-ID: <20210705091436.GE38629@C02TD0UTHF1T.local>
Date:   Mon, 5 Jul 2021 10:14:36 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Bharat Bhushan <bbhushan2@...vell.com>
Cc:     catalin.marinas@....com, will@...nel.org,
        daniel.lezcano@...aro.org, maz@...nel.org,
        konrad.dybcio@...ainline.org, saiprakash.ranjan@...eaurora.org,
        robh@...nel.org, marcan@...can.st, suzuki.poulose@....com,
        broonie@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Linu Cherian <lcherian@...vell.com>
Subject: Re: [PATCH] clocksource: Add Marvell Errata-38627 workaround

On Mon, Jul 05, 2021 at 10:07:53AM +0100, Mark Rutland wrote:
> Hi Bharat,
> 
> On Mon, Jul 05, 2021 at 11:38:43AM +0530, Bharat Bhushan wrote:
> > CPU pipeline have unpredicted behavior when timer
> > interrupt appears and then disappears prior to the
> > exception happening. Time interrupt appears on timer
> > expiry and disappears when timer programming or timer
> > disable. This typically can happen when a load
> > instruction misses in the cache,  which can take
> > few hundreds of cycles, and an interrupt appears
> > after the load instruction starts executing but
> > disappears before the load instruction completes.
> 
> Could you elaborate on the scenario? What sort of unpredictable
> behaviour can occur? e.g:
> 
> * Does the CPU lockup?
> * Does the CPU take the exception at all?
> * Does the load behave erroneously?
> * Does any CPU state (e.g. GPRs, PC, PSTATE) become corrupted?
> 
> Does the problem manifest when IRQs are masked by DAIF.I, or by
> CNT8_CTL_EL0.{IMASK,ENABLE} ?

Whoops, that was supposed to say:

| CNT*_CTL_EL0.{IMASK,ENABLE}

... i.e. those fields in either CNTP_CTL_EL0 or CNTV_CTL_EL0.

Thanks,
Mark.