linux-kernel - Re: [tip: x86/fpu] x86/fpu: Use XSAVE{,OPT,C,S} and XRSTOR{,S} mnemonics in xstate.h

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Message-ID: <1886668D-E44A-4510-B31E-933545FA2C23@zytor.com>
Date: Mon, 17 Mar 2025 11:38:12 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Uros Bizjak <ubizjak@...il.com>, Borislav Petkov <bp@...en8.de>
CC: linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Ingo Molnar <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org
Subject: Re: [tip: x86/fpu] x86/fpu: Use XSAVE{,OPT,C,S} and XRSTOR{,S} mnemonics in xstate.h

On March 17, 2025 4:06:11 AM PDT, Uros Bizjak <ubizjak@...il.com> wrote:
>On Mon, Mar 17, 2025 at 11:46 AM Borislav Petkov <bp@...en8.de> wrote:
>>
>> On Mon, Mar 17, 2025 at 11:28:58AM +0100, Uros Bizjak wrote:
>> > > > @@ -114,10 +113,10 @@ static inline int update_pkru_in_sigframe(struct xregs_state __user *buf, u64 ma
>> > > >  #define XSTATE_OP(op, st, lmask, hmask, err)                         \
>> > > >       asm volatile("1:" op "\n\t"                                     \
>> > > >                    "xor %[err], %[err]\n"                             \
>> > > > -                  "2:\n\t"                                           \
>> > > > +                  "2:\n"                                             \
>> > > >                    _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_FAULT_MCE_SAFE)  \
>> > > >                    : [err] "=a" (err)                                 \
>> > > > -                  : "D" (st), "m" (*st), "a" (lmask), "d" (hmask)    \
>> > > > +                  : [xa] "m" (*(st)), "a" (lmask), "d" (hmask)       \
>> > >
>> > > This [xa] needs documenting in the comment above this.
>> > >
>> > > What does "xa" even mean?
>> >
>> > xsave area.
>>
>> That's struct xregs_state in kernel nomenclature.
>>
>> And the macro's argument is called "st".
>>
>> And when it says [xa] there, one wonders where that "xa" comes from. So please
>> add a comment above the macro explaining that.
>
>This is an internal label for a named argument. The name shouldn't
>bother anybody, it could be anything, [xa], [ptr], [arg] or whatnot,
>so I see no reason why a comment should explain the choice. It's like
>arguing about the name of a variable.
>
>Uros.
>

Ok, I'm going to argue, but only because the argument is called "st" and the assembly parameter "xa". That's needlessly different and means having to look extra hard 

We can obviously not use the same token, but IMO it would make a lot more sense to call one of them _st or perhaps st_p (being a pointer).