Len,
On Thu, May 20 2021 at 17:22, Len Brown wrote:
On Thu, May 20, 2021 at 4:54 PM Thomas Gleixner [off-list ref] wrote:
quoted
quoted
AMX is analogous to the multiplier used by AVX-512.
The architectural state must exist on every CPU, including HT siblings.
Today, the HT siblings share the same execution unit,
and I have no reason to expect that will change.
I'm well aware that HT siblings share the same execution unit for
AVX.
Though AMX is if I remember the discussions two years ago correctly
shared by more than the HT siblings which makes things worse.
I regret that we were unable to get together in the last year to have
an updated discussion. I think if we had, then we would have saved
a lot of mis-understanding and a lot of email!
So let me emphasize here:
There is one TMUL execution unit per core.
It is shared by the HT siblings within that core.
So the comparison to the AVX-512 multiplier is a good one.
Fine, but that does not at all change the facts that:
1) It's shared between logical CPUs
2) It has effects on power/thermal and therefore effects which reach
outside of the core scope
3) Your appproach of making it unconditionlly available via the
proposed #NM prevents the OS and subsequently the system admin /
system designer to implement fine grained control over that
resource.
And no, an opt-in approach by providing a non-mandatory
preallocation prctl does not solve that problem.
Thanks,
tglx