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

[Borislav Petkov]

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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