linux-kernel - Re: [PATCH 08/10] arm64: mops: handle single stepping after MOPS exception

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Date:   Fri, 17 Mar 2023 16:02:06 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Kristina Martsenko <kristina.martsenko@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Mark Rutland <mark.rutland@....com>,
        Mark Brown <broonie@...nel.org>,
        Luis Machado <luis.machado@....com>,
        Vladimir Murzin <vladimir.murzin@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/10] arm64: mops: handle single stepping after MOPS
 exception

On Thu, Feb 16, 2023 at 04:00:10PM +0000, Kristina Martsenko wrote:
> When a MOPS main or epilogue instruction is being executed, the task may
> get scheduled on a different CPU and restart execution from the prologue
> instruction. If the main or epilogue instruction is being single stepped
> then it makes sense to finish the step and take the step exception
> before starting to execute the next (prologue) instruction. So
> fast-forward the single step state machine when taking a MOPS exception.
> 
> This means that if a main or epilogue instruction is single stepped with
> ptrace, the debugger will sometimes observe the PC moving back to the
> prologue instruction. (As already mentioned, this should be rare as it
> only happens when the task is scheduled to another CPU during the step.)
> 
> This also ensures that perf breakpoints count prologue instructions
> consistently (i.e. every time they are executed), rather than skipping
> them when there also happens to be a breakpoint on a main or epilogue
> instruction.
> 
> Signed-off-by: Kristina Martsenko <kristina.martsenko@....com>

Acked-by: Catalin Marinas <catalin.marinas@....com>