RE: [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable

quoted

No #MC on stores. Just on loads. Note that you can't clear poison
state with a series of small writes to the cache line. But a single
64-byte store might do it (architects didn't want to guarantee that
it would work when I asked about avx512 stores to clear poison
many years ago).

Dave Jiang thinks MOVDIR64B clears poison.

http://archive.lwn.net:8080/linux-kernel/157617505636.42350.1170110675242558018.stgit@djiang5-desk3.ch.intel.com/

MOVDIR64B has some explicit guarantees (does a write-back invalidate if the target is already
in the cache) that a 64-byte avx512 write doesn't.

Of course it would stop working if some future CPU were to have a longer than 64 bytes cache line.

-Tony