From: Catalin Marinas <catalin.marinas@arm.com>
To: Jisheng Zhang <jszhang@kernel.org>
Cc: Will Deacon <will@kernel.org>,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] arm64: runtime-const: save one instruction when ARM64_VA_BITS <= 48
Date: Fri, 27 Feb 2026 16:34:04 +0000 [thread overview]
Message-ID: <aaHHfCL4hdV4z9Uo@arm.com> (raw)
In-Reply-To: <20260225144613.30846-1-jszhang@kernel.org>
On Wed, Feb 25, 2026 at 10:46:13PM +0800, Jisheng Zhang wrote:
> Currently, the runtime_const_ptr() uses 4 instructions to move a long
> imm to GP, but when ARM64_VA_BITS <= 48(which is true for android and
> armbian), the top 8bits of runtime cont ptr is all '1', so we can make
^^^^^
8 or 16?
> use of the movn instruction to construct the imm's top 8bits and lower
> 16bits at the same time, thus save one instruction.
This works as long as KASAN_{SW,HW}_TAGS is disabled, otherwise the top
byte of a pointer is not guaranteed to be 0xff. I think both
filename_init() and dcache_init() can pass tagged pointers.
> diff --git a/arch/arm64/include/asm/runtime-const.h b/arch/arm64/include/asm/runtime-const.h
> index be5915669d23..6797dd37d690 100644
> --- a/arch/arm64/include/asm/runtime-const.h
> +++ b/arch/arm64/include/asm/runtime-const.h
> @@ -7,6 +7,8 @@
> /* Sigh. You can still run arm64 in BE mode */
> #include <asm/byteorder.h>
>
> +#if CONFIG_ARM64_VA_BITS > 48
You could use VA_BITS, it's shorter, though if you add the KASAN checks
it's a pretty long #if to copy all over the place. We could untag the
pointer but it kind of defeats the purpose of enabling KASAN in the
first place.
Given that Android enables KASAN_HW_TAGS by default, not sure we should
bother with this change. Do you have any perf data to show that it's
worth it?
--
Catalin
next prev parent reply other threads:[~2026-02-27 16:34 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-02-25 14:46 [PATCH] arm64: runtime-const: save one instruction when ARM64_VA_BITS <= 48 Jisheng Zhang
2026-02-27 16:34 ` Catalin Marinas [this message]
2026-03-02 16:08 ` Jisheng Zhang
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=aaHHfCL4hdV4z9Uo@arm.com \
--to=catalin.marinas@arm.com \
--cc=jszhang@kernel.org \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.