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Message-ID: <Z9frKMLYnhZI0MDD@gmail.com>
Date: Mon, 17 Mar 2025 10:28:08 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Borislav Petkov <bp@...en8.de>
Cc: Uros Bizjak <ubizjak@...il.com>, x86@...nel.org,
linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"H. Peter Anvin" <hpa@...or.com>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/asm: Use asm_inline() instead of asm() in
__untagged_addr()
* Ingo Molnar <mingo@...nel.org> wrote:
> But GCC cannot always do proper inlining decisions due to our
> complicated ALTERNATIVE macro constructs confusing the GCC inliner:
>
> > > ALTERNATIVE macro that expands to several pseudo directives causes
> > > instruction length estimate to count more than 20 instructions.
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> Note how the asm_inline() compiler feature was added by GCC at the
> kernel community's request to address such issues. (!)
>
> So for those reasons, in my book, eliminating a function call for
> really simple single instruction inlines is an unconditional
> improvement that doesn't require futile performance measurements - it
> 'only' requires assembly level code generation analysis in the
> changelog.
Note that at least in part this is a weakness of GCC: the compiler
isn't looking at the asm() closely enough and the 20 instructions count
vastly overestimates the true footprint of these statements.
Yet GCC is also giving us a tool: "asm __inline", which tells the
compiler that this piece of asm() statement is small. A tool that was
created at the request of the kernel community's complaints about this
issue. :-/
asm_inline() is functionally similar to __force_inline - which we
regularly apply if it has code generation benefits.
So I really don't see the harm in these patches - they have benefits in
terms of GCC code generation quality, documentation and performance:
- It documents small asm() statements by annotating them asm_inline().
- It sometimes avoids function call overhead, improving performance.
And because single-function inlining changes are next to impossible to
measure in practice in most cases, I'd suggest we skip the performance
measurement requirement if the code generation advantages on a recent
GCC version are unambiguous.
Thanks,
Ingo
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