linux-kernel - Re: [GIT PULL] KVM fixes for Linux 5.15-rc7

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Date:   Mon, 18 Oct 2021 08:15:05 -1000
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Jim Mattson <jmattson@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>
Subject: Re: [GIT PULL] KVM fixes for Linux 5.15-rc7

On Mon, Oct 18, 2021 at 8:03 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> The code is not wrong, there is a comment explaining it:
>
>          * Use a bitwise-OR instead of a logical-OR to aggregate the reserved
>          * bits and EPT's invalid memtype/XWR checks to avoid an extra Jcc
>          * (this is extremely unlikely to be short-circuited as true).

That makes very little sense.

It seems to be avoiding a 'jcc' and replace it with a 'setcc' and an
'or'. Which is likely both bigger and slower.

If the jcc were unpredictable, maybe that would be one thing. But at
least from a quick look, that doesn't even seem likely

 The other use of that function has a "WARN_ONCE()" on it, so
presumably it normally doesn't ever trigger either of the boolean
conditions.

Very strange code.

                    Linus