linux-kernel - Re: [PATCH tip] x86/percpu: Rewrite arch_raw_cpu

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Message-ID: <ZSpnpfn/mSYrgC9C@gmail.com>
Date:   Sat, 14 Oct 2023 12:04:21 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Nadav Amit <namit@...are.com>,
        Andy Lutomirski <luto@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH tip] x86/percpu: Rewrite arch_raw_cpu_ptr()


* Uros Bizjak <ubizjak@...il.com> wrote:

> Implement arch_raw_cpu_ptr() as a load from this_cpu_off and then
> add the ptr value to the base. This way, the compiler can propagate
> addend to the following instruction and simplify address calculation.
> 
> E.g.: address calcuation in amd_pmu_enable_virt() improves from:
> 
>     48 c7 c0 00 00 00 00 	mov    $0x0,%rax
> 	87b7: R_X86_64_32S	cpu_hw_events
> 
>     65 48 03 05 00 00 00 	add    %gs:0x0(%rip),%rax
>     00
> 	87bf: R_X86_64_PC32	this_cpu_off-0x4
> 
>     48 c7 80 28 13 00 00 	movq   $0x0,0x1328(%rax)
>     00 00 00 00
> 
> to:
> 
>     65 48 8b 05 00 00 00 	mov    %gs:0x0(%rip),%rax
>     00
> 	8798: R_X86_64_PC32	this_cpu_off-0x4
>     48 c7 80 00 00 00 00 	movq   $0x0,0x0(%rax)
>     00 00 00 00
> 	87a6: R_X86_64_32S	cpu_hw_events+0x1328
> 
> The compiler can also eliminate redundant loads from this_cpu_off,
> reducing the number of percpu offset reads (either from this_cpu_off
> or with rdgsbase) from 1663 to 1571.
> 
> Additionaly, the patch introduces 'rdgsbase' alternative for CPUs with
> X86_FEATURE_FSGSBASE. The rdgsbase instruction *probably* will end up
> only decoding in the first decoder etc. But we're talking single-cycle
> kind of effects, and the rdgsbase case should be much better from
> a cache perspective and might use fewer memory pipeline resources to
> offset the fact that it uses an unusual front end decoder resource...

So the 'additionally' wording in the changelog should have been a big hint 
already that the introduction of RDGSBASE usage needs to be a separate 
patch. ;-)

Thanks,

	Ingo