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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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