From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id C82EF289368; Wed, 9 Jul 2025 10:09:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752055782; cv=none; b=tyQtGC1ueBjOdjIsp0ENGcx5O5sPYAqUex8LAQv7sBsIGPetZA1RpM6jzNpoCqbSWWa2ZoIKx/FG2nZtm3/EfTXBQ3ta79fBOQJwKMJB6AExXyXTBui1Oe7SIWO3SkSn/+VB7kAKEA5s63OaeKxJdVsdWhI1hn0Q7guM4cutOYI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752055782; c=relaxed/simple; bh=Sw7oI7H4anCTEdENQ9OK36GMbuYvdgqWc1bZLlsHgW0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=uiLtpsgBhN8ORmSWts6pyMf0uh2196YR5XhdMdGz1cvc5t/1HTuPqKSQf2LZtB8VK+dg35Z2WAR+4gs2tlqBeuuwbZhUYOoYoBVf8ML1ciwL6SimTZq6V/ccyIAsu/D1h62vOw3iwe54b89/UHrAH0y9ZwWLS2xwH6KR+CqKd/w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 9060F153B; Wed, 9 Jul 2025 03:09:28 -0700 (PDT) Received: from raptor (usa-sjc-mx-foss1.foss.arm.com [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 82CB43F694; Wed, 9 Jul 2025 03:09:38 -0700 (PDT) Date: Wed, 9 Jul 2025 11:09:32 +0100 From: Alexandru Elisei To: James Clark Cc: Will Deacon , Mark Rutland , Catalin Marinas , Anshuman Khandual , Rob Herring , Suzuki Poulose , Robin Murphy , linux-arm-kernel@lists.infradead.org, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/3] perf: arm_spe: Disable buffer before writing to PMBPTR_EL1 or PMBSR_EL1 Message-ID: References: <20250701-james-spe-vm-interface-v1-0-52a2cd223d00@linaro.org> <20250701-james-spe-vm-interface-v1-2-52a2cd223d00@linaro.org> Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250701-james-spe-vm-interface-v1-2-52a2cd223d00@linaro.org> Hi James, On Tue, Jul 01, 2025 at 04:31:58PM +0100, James Clark wrote: > DEN0154 states that writes to PMBPTR_EL1 or PMBSR_EL1 must be done while > the buffer is disabled (PMBLIMITR_EL1.E == 0). Re-arrange the interrupt > handler to always disable the buffer for non-spurious interrupts before > doing either. > > Most of arm_spe_pmu_disable_and_drain_local() is now always done, so for > faults the only thing left to do is clear PMSCR_EL1. > > Elaborate the comment in arm_spe_pmu_disable_and_drain_local() to > explain the ramifications of not doing it in the right order. > > Fixes: d5d9696b0380 ("drivers/perf: Add support for ARMv8.2 Statistical Profiling Extension") > Signed-off-by: James Clark > --- > drivers/perf/arm_spe_pmu.c | 33 +++++++++++++++++++++------------ > 1 file changed, 21 insertions(+), 12 deletions(-) > > diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c > index 6235ca7ecd48..5829947c8871 100644 > --- a/drivers/perf/arm_spe_pmu.c > +++ b/drivers/perf/arm_spe_pmu.c > @@ -559,7 +559,12 @@ static void arm_spe_perf_aux_output_end(struct perf_output_handle *handle) > > static void arm_spe_pmu_disable_and_drain_local(void) > { > - /* Disable profiling at EL0 and EL1 */ > + /* > + * To prevent the CONSTRAINED UNPREDICTABLE behavior of either writing > + * to memory after the buffer is disabled, or SPE reporting an access > + * not allowed event, we must disable sampling before draining the > + * buffer. > + */ > write_sysreg_s(0, SYS_PMSCR_EL1); > isb(); > > @@ -661,16 +666,24 @@ static irqreturn_t arm_spe_pmu_irq_handler(int irq, void *dev) > */ > irq_work_run(); > > + /* > + * arm_spe_pmu_buf_get_fault_act() already drained, and PMBSR_EL1.S == 1 > + * means that StatisticalProfilingEnabled() == false. So now we can > + * safely disable the buffer. > + */ > + write_sysreg_s(0, SYS_PMBLIMITR_EL1); > + isb(); > + > + /* Status can be cleared now that PMBLIMITR_EL1.E == 0 */ > + write_sysreg_s(0, SYS_PMBSR_EL1); I've been trying to figure out if we need an ISB here to order clearing the service bit before the PMBLIMITR_EL1 write in arm_spe_perf_aux_output_begin(). arm_spe_perf_aux_output_begin() is called only when the buffer is full, and this rules out the event having the discard attribute (buffer management events are not generated in discard mode). If a new buffer cannot be allocated (perf_aux_output_begin() returns NULL), then PMBLIMITR_EL1.E remains 0, so no need to order the two writes. The only other path remaining in arm_spe_perf_aux_output_begin() is reprogramming the buffer, in which case there's an ISB before the write to PMBLIMITR_EL1. In conclusion, I think it's correct not to have an ISB here. > + > switch (act) { > case SPE_PMU_BUF_FAULT_ACT_FATAL: > /* > - * If a fatal exception occurred then leaving the profiling > - * buffer enabled is a recipe waiting to happen. Since > - * fatal faults don't always imply truncation, make sure > - * that the profiling buffer is disabled explicitly before > - * clearing the syndrome register. > + * To complete the full disable sequence, also disable profiling > + * at EL0 and EL1, we don't want to continue at all anymore. > */ > - arm_spe_pmu_disable_and_drain_local(); > + write_sysreg_s(0, SYS_PMSCR_EL1); Before: arm_spe_pmu_buf_get_fault_act: ISB arm_spe_pmu_disable_and_drain_local: PMSCR_EL1 = 0 ISB # disables profiling PMBLIMITR_EL1=0 PMBSR_EL1=0 ERET # synchronizes the two writes above Now: arm_spe_pmu_buf_get_fault_act: ISB PMBLIMITR_EL1=0 ISB # disables profiling PMBSR_EL1=0 PMSCR_EL1=0 ERET # synchronizes the two writes above This looks correct to me. Thanks, Alex > break; > case SPE_PMU_BUF_FAULT_ACT_OK: > /* > @@ -679,18 +692,14 @@ static irqreturn_t arm_spe_pmu_irq_handler(int irq, void *dev) > * PMBPTR might be misaligned, but we'll burn that bridge > * when we get to it. > */ > - if (!(handle->aux_flags & PERF_AUX_FLAG_TRUNCATED)) { > + if (!(handle->aux_flags & PERF_AUX_FLAG_TRUNCATED)) > arm_spe_perf_aux_output_begin(handle, event); > - isb(); > - } > break; > case SPE_PMU_BUF_FAULT_ACT_SPURIOUS: > /* We've seen you before, but GCC has the memory of a sieve. */ > break; > } > > - /* The buffer pointers are now sane, so resume profiling. */ > - write_sysreg_s(0, SYS_PMBSR_EL1); > return IRQ_HANDLED; > } > > > -- > 2.34.1 >