[MODERATED] Re: Some microperf tests

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 23/02/2019 19:30, speck for Linus Torvalds wrote:
> So those numbers are actually sensible.

The old behaviour has always made sense in terms of how legacy software
was expected to use it.

However, given the recommendation, I wasn't certain that the new
behaviour is deliberate.  My first thought was whether it was an
unintended consequence of the NUL selector not resulting in a memory
access, while the pipeline buffers are being frobbed.

> Of course, the fact that verw on a NUL descriptor is so much slower in
> the old case is very inconvenient for the "we should do verw even if
> the CPU says it doesn't have the microcode update, for vmware rasons",
> aka vmverw.
>
> So it would be good to verify that
>
>  (a) yes, this is the intended performance profile from intel
>
>  (b) we probably should give a NUL descriptor for the workaround

As I said - I've currently got these questions open with the relevant
people.  I'll feed back the reply.

>  (c) it hurts the vmverw case, but maybe we can only do vmverw when we
> notice we are actually running under vmware.
>
> Is there any way to do that vmware detection?

CPUID leaves at 0x40000000?  Not necessarily perfect, but will cover the
overwhelming majority of cases.

~Andrew