On 18/09/2019 10:22, Jan Beulich wrote:
> On 17.09.2019 21:00, Andrew Cooper wrote:
>> On 17/09/2019 07:14, Jan Beulich wrote:
>>> I can't see any technical or performance reason why we should treat
>>> 32-bit PV different from 64-bit PV in this regard.
>> Well, other than the fact this setting is only read for a 64bit guest...
> How come? make_cr3() uses it uniformly, as does pv_make_cr4().
> toggle_guest_mode() is the one case where it's strictly 64-bit
> guest only.
Oh - you are right. I don't know how I came to the above conclusion,
but ...
>> The reason it isn't set for 32bit guests is that there is no scenario
>> where we use it.
> "pcid=1" and "pcid=noxpti" both are scenarios where, with this
> patch in place, we would use it.
... I still don't see why this is sensible.
As far as I can tell, all it will do for a 32bit PV guest is start using
2 PCIDs for the same logical gCR3, which will be a net perf it hit for
32bit PV guests.
This is ultimately wrapped up in the confusion over TF_kernel_mode and
v->arch.guest_table{,_user}.
~Andrew
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