Message-ID: <de01a4c227870c4cf52370aa00bc7b8ddf136da9.camel@infradead.org>
Subject: Re: [PATCH v12 07/11] x86/smpboot: Remove early_gdt_descr on 64-bit
From:   David Woodhouse <dwmw2@infradead.org>
To:     "H. Peter Anvin" <hpa@zytor.com>, Brian Gerst <brgerst@gmail.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Usama Arif <usama.arif@bytedance.com>, kim.phillips@amd.com,
        piotrgorski@cachyos.org, oleksandr@natalenko.name,
        arjan@linux.intel.com, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, x86@kernel.org, pbonzini@redhat.com,
        paulmck@kernel.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, rcu@vger.kernel.org, mimoja@mimoja.de,
        hewenliang4@huawei.com, thomas.lendacky@amd.com, seanjc@google.com,
        pmenzel@molgen.mpg.de, fam.zheng@bytedance.com,
        punit.agrawal@bytedance.com, simon.evans@bytedance.com,
        liangma@liangbit.com
Date:   Wed, 01 Mar 2023 07:39:39 +0000
In-Reply-To: <CE9D5D21-63D3-40B8-BC71-11F1A5A93F7E@zytor.com>
References: <20230226110802.103134-1-usama.arif@bytedance.com>
         <20230226110802.103134-8-usama.arif@bytedance.com> <878rghmrn2.ffs@tglx>
         <35264451eabdf967eb31069cb814e8a05ee3179b.camel@infradead.org>
         <cf7a1657159219cea61ffef567280e2e88d1f670.camel@infradead.org>
         <CAMzpN2hQArxf2mAVq55uMx9VhTjUD-VDEVAD406RScfsrjdAjQ@mail.gmail.com>
         <0E3AFDDA-039B-40F3-92A4-85CD96B1BB64@infradead.org>
         <CE9D5D21-63D3-40B8-BC71-11F1A5A93F7E@zytor.com>
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On Tue, 2023-02-28 at 16:02 -0800, H. Peter Anvin wrote:
>=20
> > Ewww. Couldn't the assembler choose to omit the REX prefix then? It
> > does more tricksy things than that already.
> >=20
> > I almost prefer having the prefix but (in the morning) if you
> > prefer I can put it back as it was with a comment about the zero-
> > extension.
> >=20
>=20
> Like it or not, that's how the assembler currently works.

Last time someone told me "you can't tell the assembler what you
actually mean; you have to tell it something different" I threw my toys
out of the pram a little bit and implemented all of the 16-bit '-m16'
support in LLVM/clang.

On Tue, 2023-02-28 at 19:03 -0500, Brian Gerst wrote:
>=20
> commit a7bea8308933aaeea76dad7d42a6e51000417626
> Author: Jan Beulich <JBeulich@suse.com>
> Date:   Mon Jul 2 04:31:54 2018 -0600
>=20
>     x86/asm/64: Use 32-bit XOR to zero registers
>=20
>     Some Intel CPUs don't recognize 64-bit XORs as zeroing idioms. Zeroin=
g
>     idioms don't require execution bandwidth, as they're being taken care
>     of in the frontend (through register renaming). Use 32-bit XORs inste=
ad.
>=20
> Not that speed is important here, but it's good to be consistent
> across the whole kernel.  Someone will eventually come by and fix it
> up anyways, as there have been a few of these patches already in the
> git history.

So there literally is a better encoding of this mnemonic which isn't
just a byte shorter, but is also faster to execute. And one *hopes*
that the compiler knows about it... yep... so why in $DEITY's name
can't I type what I mean into an asm file? Isn't asm intricate enough
already?

Meh, life's too short. Not shaving that particular yak today. I've put
the git tree back to using %edx, with a comment that it zero-extends.


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