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

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Date:   Thu, 14 Jul 2022 10:02:57 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Boris Petkov <bp@...en8.de>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Peter Zijlstra <peterz@...radead.org>,
        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

On Thu, Jul 14, 2022 at 7:46 AM Boris Petkov <bp@...en8.de> wrote:
>
> On July 14, 2022 1:46:53 PM UTC, Paolo Bonzini <pbonzini@...hat.com> wrote:
> >Please leave that one out as Peter suggested a better fix and I have that queued for Linus.
>
> Already zapped.

I like Peter's more obvious use of FASTYOP_LENGTH, but this is just disgusting:

    #define FASTOP_SIZE (8 << ((FASTOP_LENGTH > 8) & 1) <<
((FASTOP_LENGTH > 16) & 1))

I mean, I understand what it's doing, but just two lines above it the
code has a "ilog2()" use that already depends on the fact that you can
use ilog2() as a constant compile-time expression.

And guess what? The code could just use roundup_pow_of_two(), which is
designed exactly like ilog2() to be used for compile-time constant
values.

So the code should just use

    #define FASTOP_SIZE roundup_pow_of_two(FASTOP_LENGTH)

and be a lot more legible, wouldn't it?

Because I don't think there is anything magical about the length
"8/16/32". It's purely "aligned and big enough to contain
FASTOP_LENGTH".

And then the point of that

    static_assert(FASTOP_LENGTH <= FASTOP_SIZE);

just goes away, because there are no subtle math issues there any more.

In fact, the remaining question is just "where did the 7 come from" in

    #define FASTOP_LENGTH (7 + ENDBR_INSN_SIZE + RET_LENGTH)

because other than that it all looks fairly straightforward.

              Linus