linux-kernel - Re: FYI: path walking optimizations pending for 6.11

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Message-ID: <Zo0LECcBUElkHPGs@J2N7QTR9R3>
Date: Tue, 9 Jul 2024 11:04:00 +0100
From: Mark Rutland <mark.rutland@....com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Christian Brauner <brauner@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux ARM <linux-arm-kernel@...ts.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: FYI: path walking optimizations pending for 6.11

Hi Linus,

On Wed, Jun 19, 2024 at 01:25:02PM -0700, Linus Torvalds wrote:
> I've pushed out four branches based on 6.10-rc4, because I think it's
> pretty ready. But I'll rebase them if people have commentary that
> needs addressing, so don't treat them as some kind of stable base yet.
> My plan is to merge them during the next merge window unless somebody
> screams.
> 
> The branches are:
> 
> arm64-uaccess:
>     arm64: access_ok() optimization
>     arm64: start using 'asm goto' for put_user()
>     arm64: start using 'asm goto' for get_user() when available

> runtime-constants:
>     arm64: add 'runtime constant' support
>     runtime constants: add x86 architecture support
>     runtime constants: add default dummy infrastructure
>     vfs: dcache: move hashlen_hash() from callers into d_hash()

Apologies, the arm64 branches/patches have been on my TODO list for
review/test/benchmark for the last couple of weeks, but I haven't had
both the time and machine availability to do so.

Looking at the arm64 runtime constants patch, I see there's a redundant
store in __runtime_fixup_16(), which I think is just a leftover from
applying the last roudn or feedback:

+/* 16-bit immediate for wide move (movz and movk) in bits 5..20 */
+static inline void __runtime_fixup_16(__le32 *p, unsigned int val)
+{
+	u32 insn = le32_to_cpu(*p);
+	insn &= 0xffe0001f;
+	insn |= (val & 0xffff) << 5;
+	*p = insn;
+	*p = cpu_to_le32(insn);
+}

... i.e. the first assignment to '*p' should go; the compiler should be
smart enough to elide it entirely, but it shouldn't be there.

For the sake of review, would you be happy to post the uaccess and
runtime-constants patches to the list again? I think there might be some
remaining issues with (real) PAN and we might need to do a bit more
preparatory work there.

Mark.