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Wed, 12 May 2021 13:07:12 +0000 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 D6A3B31B; Wed, 12 May 2021 06:07:02 -0700 (PDT) Received: from C02TD0UTHF1T.local (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5ADD33F718; Wed, 12 May 2021 06:07:01 -0700 (PDT) Date: Wed, 12 May 2021 14:06:58 +0100 From: Mark Rutland To: Robin Murphy Cc: will@kernel.org, catalin.marinas@arm.com, linux-arm-kernel@lists.infradead.org, yangyingliang@huawei.com, shenkai8@huawei.com Subject: Re: [PATCH 8/8] arm64: Rewrite __arch_clear_user() Message-ID: <20210512130418.GF88854@C02TD0UTHF1T.local> References: <76a1700b0316b50fb5881da603f2daf3c81468f4.1620738177.git.robin.murphy@arm.com> <20210512104833.GD88854@C02TD0UTHF1T.local> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210512_060708_270359_9F63D91F X-CRM114-Status: GOOD ( 21.29 ) 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: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, May 12, 2021 at 12:31:39PM +0100, Robin Murphy wrote: > On 2021-05-12 11:48, Mark Rutland wrote: > > On Tue, May 11, 2021 at 05:12:38PM +0100, Robin Murphy wrote: > > > Rewrite __arch_clear_user() with regular > > > USER() annotations so that it's clearer what's going on, and take the > > > opportunity to minimise the branchiness in the most common paths, which > > > also allows the exception fixup to return a more accurate result. > > > > IIUC this isn't always accurate for the {4,2,1}-byte cases; example > > below. I'm not sure whether that's intentional since the commit message > > says "more accurate" rather than "accurate". > > Indeed, the "more" was definitely significant :) :) > > If we think that under-estimating is fine, I reckon it'd be worth a > > comment to make that clear. > > Indeed for smaller amounts there's no change in fixup behaviour at all, but > I have to assume that underestimating by up to 100% is probably OK since > we've been underestimating by fully 100% for nearly 10 years now. I don't > believe it's worth having any more complexity than necessary for the fault > case - grepping for clear_user() usage suggests that nobody really cares > about the return value beyond whether it's zero or not, so the minor > "improvement" here is more of a nice-to-have TBH. > > The existing comment doesn't actually explain anything either, which is why > I didn't replace it, but I'm happy to add something if you like. I don't have strong feelings either way, but I think that we should at least document this, since that'll at least save us rehashing the same point in future. :) That said, IIUC to make this always accurate we only need two ADDs (diff below). Since those will only be executed at most once each, I suspect they won't have a measureable impact in practice. So maybe it's worth adding them to avoid any risk that someone needs this to be accurate in future? Mark. ---->8---- diff --git a/arch/arm64/lib/clear_user.S b/arch/arm64/lib/clear_user.S index 1005345b4066..7ef553ec2677 100644 --- a/arch/arm64/lib/clear_user.S +++ b/arch/arm64/lib/clear_user.S @@ -32,12 +32,14 @@ USER(9f, sttr xzr, [x2, #-8]) 2: tbz x1, #2, 3f USER(9f, sttr wzr, [x0]) + add x0, x0, #4 USER(9f, sttr wzr, [x2, #-4]) mov x0, #0 ret 3: tbz x1, #1, 4f USER(9f, sttrh wzr, [x0]) + add x0, x0, #2 4: tbz x1, #0, 5f USER(9f, sttrb wzr, [x2, #-1]) 5: mov x0, #0 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel