linux-kernel - Re: [PATCH 1/4] x86/asm: inline constant inputs in rdpkru(), wrpkru()

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Message-ID: <360f4fcc-8141-449c-a5fc-cd91182e0899@p183>
Date: Fri, 7 Mar 2025 19:19:52 +0300
From: Alexey Dobriyan <adobriyan@...il.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] x86/asm: inline constant inputs in rdpkru(), wrpkru()

On Thu, Mar 06, 2025 at 01:35:01PM -0800, Dave Hansen wrote:
> On 3/6/25 13:07, Alexey Dobriyan wrote:
> >  static inline void wrpkru(u32 pkru)
> >  {
> > -	u32 ecx = 0, edx = 0;
> > -
> >  	/*
> >  	 * "wrpkru" instruction.  Loads contents in EAX to PKRU,
> >  	 * requires that ecx = edx = 0.
> >  	 */
> >  	asm volatile(".byte 0x0f,0x01,0xef\n\t"
> > -		     : : "a" (pkru), "c"(ecx), "d"(edx));
> > +		     : : "a" (pkru), "c" (0), "d" (0));
> >  }
> 
> Hey Alexey,
> 
> I appreciate the patch. But I do like how it's written currently. I
> honestly kinda wish it went even further and did:
> 
> 	u32 eax = pkru;

I _think_ you can write   "eax" (pkru) .

I don't like the comment either. It just reiterates the SDM.
Trailing \n\t too -- they don't do anything in one-liners.

The function is basically:

	// wrpkru
	asm volatile (
		".byte 0x0f,0x01,0xef"
		:
		: "a" (pkru), "d" (0) , "c" (0)
	);