linux-kernel - Re: [PATCH v3 2/8] x86/sev: Add support for the RMPREAD instruction

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Message-ID: <20241021154117.GFZxZ2HbUPG9ux8bYr@fat_crate.local>
Date: Mon, 21 Oct 2024 17:41:17 +0200
From: Borislav Petkov <bp@...en8.de>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Michael Roth <michael.roth@....com>,
	Ashish Kalra <ashish.kalra@....com>
Subject: Re: [PATCH v3 2/8] x86/sev: Add support for the RMPREAD instruction

On Fri, Oct 18, 2024 at 10:14:04AM -0500, Tom Lendacky wrote:
> I don't think so. RCX does not change on output, the contents that RCX
> points to changes, but the register value does not so the "+" is not
> correct. The instruction doesn't take a memory location as part of
> operands (like a MOV instruction could), which is why the "memory" clobber
> is specified.

Just confirmed it with my compiler guy: yes, you're right. The rule is this:
*if* RCX itself doesn't change but memory it points to, does change, then you
need the "memory" clobber. Otherwise the compiler can reorder accesses.

> For RAX, yes, if I set "ret" to the input value then I can use "+"
> specification. But the way it's coded now is also correct.

If you set ret, it means a smaller and simpler inline asm which is always
better.

:-)

Thx.

-- 
Regards/Gruss,
    Boris.

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