From: Mark Rutland <mark.rutland@arm.com>
To: Robin Murphy <robin.murphy@arm.com>
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()
Date: Wed, 12 May 2021 14:06:58 +0100 [thread overview]
Message-ID: <20210512130418.GF88854@C02TD0UTHF1T.local> (raw)
In-Reply-To: <b1689c7f-6c61-41ca-0976-a32ceaa7eeeb@arm.com>
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
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next prev parent reply other threads:[~2021-05-12 13:09 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-11 16:12 [PATCH 0/8] arm64: String function updates Robin Murphy
2021-05-11 16:12 ` [PATCH 1/8] arm64: Import latest version of Cortex Strings' memcmp Robin Murphy
2021-05-12 13:28 ` Mark Rutland
2021-05-12 13:38 ` Robin Murphy
2021-05-12 14:51 ` Szabolcs Nagy
2021-05-26 10:17 ` Mark Rutland
2021-05-11 16:12 ` [PATCH 2/8] arm64: Import latest version of Cortex Strings' strcmp Robin Murphy
2021-05-11 16:12 ` [PATCH 3/8] arm64: Import updated version of Cortex Strings' strlen Robin Murphy
2021-05-11 16:12 ` [PATCH 4/8] arm64: Import latest version of Cortex Strings' strncmp Robin Murphy
2021-05-11 16:12 ` [PATCH 5/8] arm64: Add assembly annotations for weak-PI-alias madness Robin Murphy
2021-05-11 16:12 ` [PATCH 6/8] arm64: Import latest memcpy()/memmove() implementation Robin Murphy
2021-05-11 16:12 ` [PATCH 7/8] arm64: Better optimised memchr() Robin Murphy
2021-05-14 14:55 ` Catalin Marinas
2021-05-14 18:38 ` Robin Murphy
2021-05-11 16:12 ` [PATCH 8/8] arm64: Rewrite __arch_clear_user() Robin Murphy
2021-05-12 10:48 ` Mark Rutland
2021-05-12 11:31 ` Robin Murphy
2021-05-12 13:06 ` Mark Rutland [this message]
2021-05-12 13:51 ` Robin Murphy
2021-05-14 11:57 ` [PATCH v2] " Robin Murphy
2021-05-26 11:15 ` Mark Rutland
2021-05-27 13:24 ` Robin Murphy
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