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Re: [PATCH] x86emul: recognize CLDEMOTE


  • To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
  • Date: Tue, 25 Jan 2022 15:09:13 +0000
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  • Thread-topic: [PATCH] x86emul: recognize CLDEMOTE

On 25/01/2022 14:22, Jan Beulich wrote:
> We claim to support the insn, but so far the emulator has been handling
> it as a NOP.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Treating CLDEMOTE as a NOP is going to be more efficient than actually
setting up the mapping to execute a real CLDEMOTE instruction on the
line in question.  CLDEMOTE is very specifically an optimisation for
software producer/consumer pairs.

If we want to take this patch, it should Fix[es]: ad3abc47dd23c which
made the claim that CLDEMOTE needed no further additions.

The only issue on whether we can treat it as a NOP completely is whether
we believe the exception list.  I'm not sure I believe the absence of
AGU faults, but the instruction is taken from hint-nop space so
guaranteed to behave similarly to clflush/clwb.

~Andrew

 


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