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Wed, 04 Sep 2024 08:29:50 +0000 (GMT) Received: from nalasex01c.na.qualcomm.com (nalasex01c.na.qualcomm.com [10.47.97.35]) by NALASPPMTA03.qualcomm.com (8.18.1.2/8.18.1.2) with ESMTPS id 4848Tn8E022676 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Wed, 4 Sep 2024 08:29:49 GMT Received: from [10.131.117.146] (10.80.80.8) by nalasex01c.na.qualcomm.com (10.47.97.35) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1544.9; Wed, 4 Sep 2024 01:29:45 -0700 Message-ID: Date: Wed, 4 Sep 2024 13:59:38 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] iommu/arm-smmu: Don't disable next-page prefetcher on devices it works on To: Doug Anderson , Will Deacon CC: Robin Murphy , Joerg Roedel , Stephen Boyd , Chen Lin , , , , References: <20240513161343.1.I5db5530070a1335e6cc3c55e056c2a84b1031308@changeid> <20240517163742.GA525@willie-the-truck> Content-Language: en-US From: Pankaj Patil In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-Originating-IP: [10.80.80.8] X-ClientProxiedBy: nasanex01b.na.qualcomm.com (10.46.141.250) To nalasex01c.na.qualcomm.com (10.47.97.35) X-QCInternal: smtphost X-Proofpoint-Virus-Version: vendor=nai engine=6200 definitions=5800 signatures=585085 X-Proofpoint-ORIG-GUID: 4JxAi8u3j0kxiKsTL1aYbT-sgNV0zrmI X-Proofpoint-GUID: 4JxAi8u3j0kxiKsTL1aYbT-sgNV0zrmI X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1039,Hydra:6.0.680,FMLib:17.12.60.29 definitions=2024-09-04_05,2024-09-04_01,2024-09-02_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 bulkscore=0 phishscore=0 impostorscore=0 malwarescore=0 lowpriorityscore=0 suspectscore=0 priorityscore=1501 adultscore=0 clxscore=1011 mlxscore=0 spamscore=0 mlxlogscore=999 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2407110000 definitions=main-2409040065 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240904_013007_301832_51815293 X-CRM114-Status: GOOD ( 30.33 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 5/17/2024 10:49 PM, Doug Anderson wrote: > Hi, > > On Fri, May 17, 2024 at 9:37 AM Will Deacon wrote: >> >> Hi Doug, >> >> On Mon, May 13, 2024 at 04:13:47PM -0700, Douglas Anderson wrote: >>> On sc7180 trogdor devices we get a scary warning at bootup: >>> arm-smmu 15000000.iommu: >>> Failed to disable prefetcher [errata #841119 and #826419], check ACR.CACHE_LOCK >>> >>> We spent some time trying to figure out how we were going to fix these >>> errata and whether we needed to do a firmware update. Upon closer >>> inspection, however, we realized that the errata don't apply to us. >>> Specifically, the errata document says that for these errata: >>> * Found in: r0p0 >>> * Fixed in: r2p2 >>> >>> ...and trogdor devices appear to be running r2p4. That means that they >>> are unaffected despite the scary warning. >>> >>> The issue is that the kernel unconditionally tries to disable the >>> prefetcher even on unaffected devices and then warns when it's unable >>> to. >>> >>> Let's change the kernel to only disable the prefetcher on affected >>> devices, which will get rid of the scary warning on devices that are >>> unaffected. As per the comment the prefetcher is >>> "not-particularly-beneficial" but it shouldn't hurt to leave it on for >>> devices where it doesn't cause problems. >>> >>> Fixes: f87f6e5b4539 ("iommu/arm-smmu: Warn once when the perfetcher errata patch fails to apply") >>> Signed-off-by: Douglas Anderson >>> --- >>> >>> drivers/iommu/arm/arm-smmu/arm-smmu-impl.c | 21 +++++++++++++-------- >>> 1 file changed, 13 insertions(+), 8 deletions(-) >> >> >> Just curious, but did you see any performance impact (good or bad) as a >> result of this patch? The next-page prefetcher has always looked a little >> naive to me and, with a tendency for tiny TLBs in some implementations, >> there's a possibility it could do more harm than good. > > This patch actually makes no difference on trogdor today other than > getting rid of the scary warning. Specifically on trogdor the > ACR.CACHE_LOCK bit seems to be set so the kernel is unable to change > the setting anyway and has never been able to. We are working on > figuring out how to fix the firmware and then we have to get a > firmware spin before we can really see any changes. I'll keep an eye > out to see if performance numbers change when the firmware uprevs. > > BTW: any idea how big of a deal these errata are? We're _just_ > finishing a firmware uprev process and there is always pushback > against kicking off a new one unless the issue is important. Given > that we've been living with this issue since devices shipped I'm going > to assume we don't need to rush a firmware update, but if this is > really scary and needs to be addressed sooner we can figure that out. > > -Doug Receiving the warning on pre-silicon platforms as well, despite being unaffected. If merged, it will help in reducing log clutter. The patch applies cleanly on the tip of linux-next, tag: next-20240904.