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Date: Tue, 18 Mar 2025 19:04:30 +0100
From: Ingo Molnar <mingo@kernel.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: linux-kernel@vger.kernel.org, Juergen Gross <jgross@suse.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	"Ahmed S . Darwish" <darwi@linutronix.de>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	John Ogness <john.ogness@linutronix.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Borislav Petkov <bp@alien8.de>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 0/5] x86/cpu: Introduce <asm/cpuid/types.h> and
 <asm/cpuid/api.h> and clean them up
Message-ID: <Z9m1rtmlk1PxGIQA@gmail.com>
References: <20250317221824.3738853-1-mingo@kernel.org>
 <5A417EE0-8DF3-4C6E-A7E9-9EE6705282F1@zytor.com>
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In-Reply-To: <5A417EE0-8DF3-4C6E-A7E9-9EE6705282F1@zytor.com>


* H. Peter Anvin <hpa@zytor.com> wrote:

> It would be nice to get rid of the bleacherous use of *eax and *ecx 
> as input-output operands. The use of four separate pointers is just 
> barely tolerable because the compiler can remove them when the asm is 
> inlined.

So we have a nice structure of:

 struct cpuid_regs {
         u32 eax;
         u32 ebx;
         u32 ecx;
         u32 edx;
 };

So instead of:

 static inline void cpuid_count(unsigned int op, int count,
                               unsigned int *eax, unsigned int *ebx,
                               unsigned int *ecx, unsigned int *edx)

... we could have:

 static inline void cpuid_count(unsigned int op, int count, struct cpuid_regs *cregs)

or so?

plus we could implement the main CPUID call as:

 static inline void native_cpuid(struct cpuid_regs *cregs)
 {
        /* ecx is often an input as well as an output. */
        asm volatile("cpuid"
            : "=a" (cregs->eax),
              "=b" (cregs->ebx),
              "=c" (cregs->ecx),
              "=d" (cregs->edx)
            : "0" (cregs->eax), "2" (cregs->ecx)
            : "memory");
 }

and thus we give the asm() statement only a single pointer in essence, 
'cregs'?

Or do you mean something else?

Thanks,

	Ingo