cascade lake doesn't support CLWB?

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Wentao Huang

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Nov 1, 2021, 8:57:02 AM11/1/21
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I am wondering if all cascade lake CPUs can not support CLWB?
I know that some CPUs simply take CLWB as a wrapper for CLFLUSHOPT. In paper, An Empirical Guide to the Behavior and Use of Scalable Persistent Memory, the authors even test CLWB on Xeon Platinum 8160, which is skylake CPU, the predecessor of Cascade lake.
If Cascade lake does not support CLWB, I assume skylake either does not support CLWB.
So is it true that all cascade lake CPU don't support CLWB?
Thanks.

Andy Rudoff

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Nov 1, 2021, 10:29:31 AM11/1/21
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Hi,

PMem is not supported on Sky Lake.  On all Cascade Lake CPUs, CLWB never leaves the cache line valid, so it is equivalent to CLFLUSHOPT on that CPU.  We introduced the CLWB opcode in Cascade Lake so that applications and libraries could begin using it and when they encountered future CPUs they would get the benefit of CLWB leaving the cache line valid without having to rebuild.  On Ice Lake, CLWB leaves the line valid after flushing any dirty data.

-andy
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