linux-kernel - Re: [PATCH 5.15 00/78] 5.15.55-rc1 review

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Message-ID: <CAHk-=whYia1fnjJFiJ59xZv4ROqqTfG4crQNWxb71JYJf5B-Lg@mail.gmail.com>
Date:   Thu, 14 Jul 2022 11:16:40 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Boris Petkov <bp@...en8.de>, Paolo Bonzini <pbonzini@...hat.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Thadeu Lima de Souza Cascardo <cascardo@...onical.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        kvm list <kvm@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        stable <stable@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Pavel Machek <pavel@...x.de>,
        Jon Hunter <jonathanh@...dia.com>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Slade Watkins <slade@...dewatkins.com>, patches@...nelci.org,
        Sean Christopherson <seanjc@...gle.com>,
        Shuah Khan <shuah@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
        lkft-triage@...ts.linaro.org,
        Alex Bennée <alex.bennee@...aro.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Anders Roxell <anders.roxell@...aro.org>
Subject: Re: [PATCH 5.15 00/78] 5.15.55-rc1 review

Oh, btw, how bad would it be to just do

     #define FASTOP_SIZE 16
     static_assert(FASTOP_SIZE >= FASTOP_LENGTH)

and leave it at that?

Afaik both gcc and clang default to -falign-functions=16 *anyway*, and
while on 32-bit x86 we have options to minimize alignment, we don't do
that on x86-64 afaik.

In fact, we have an option to force *bigger* alignment
(DEBUG_FORCE_FUNCTION_ALIGN_64B) but not any way to make it less.

And we use

        .p2align 4

in most of our asm, aling with

    #define __ALIGN          .p2align 4, 0x90

So all the *normal* functions already get 16-byte alignment anyway.

So yeah, it would be less dense, but do we care? Wouldn't the "this is
really simple" be a nice thing? It's not like there are a ton of those
fastop functions anyway. 128 of them? Plus 16 of the "setCC" ones?

               Linus