Re: [Xen-devel] [PATCH 1/3] x86: avoid flush IPI when possible

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 10/02/16 12:56, Jan Beulich wrote:
> Since CLFLUSH, other than WBINVD, is a coherency domain wide flush,

I can't parse this sentence.

CLFUSH states "Invalidates from every level of the cache hierarchy in
the cache coherence domain"

WBINVD however states "The instruction then issues a special-function
bus cycle that directs external caches to also write back modified data
and another bus cycle to indicate that the external caches should be
invalidated."

I think we need input from Intel and AMD here as to the behaviour and
terminology here, and in particular, where the coherency domain
boundaries are.  All CPUs, even across multiple sockets, see coherent
caching, but it is unclear whether this qualifies them to be in the same
cache coherency domain per the instruction spec.

In particular, given the architecture of 8-socket systems and 45MB of
RAM in L3 caches, does wbinvd seriously drain all caches everywhere? 
Causing 45MB of data to move to remote memory controllers all at once
would cause a massive system stall.

~Andrew

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