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Re: [PATCH v2] tools/libxl: make default of max event channels dependant on vcpus

Hi Ian,

On 06/04/2020 11:47, Ian Jackson wrote:
Jürgen Groß writes ("Re: [PATCH v2] tools/libxl: make default of max event channels 
dependant on vcpus"):
On 06.04.20 11:24, Julien Grall wrote:
Large guests on which arch? Which type of guests?

I'm pretty sure this applies to x86 only. I'm not aware of event
channels being used on ARM for IPIs.

Should this be arch-dependent then ?  It seems like the figure is just
a heuristic anyway, and ...

The resulting number would be larger than today only for guests with
more than 96 vcpus. So I don't think the additional amount of memory
is really that problematic.

Julien, are there likely to be any ARM guests now which have anywhere
near that number of vcpus ?  If not do we know now what such guests
are likely to be like ?

We are meant to support up to 128 vCPUs. But our implementation can support up to 4096 vCPUs on vGICv3.

If this is all hypothetical on ARM it would seem silly to make this
arch-specific for the benefit of ARM given that the ARM implementation
would be entirely guesswork.  Maybe we should postpone that
specialisation until we know better what the ARM function should be
like for these large numbers of vcpus.

There are no correlation between event channels and vCPUs. The number of event channels only depends on the number of frontend you have in your guest. So...

If ARM folks want to have a different formula for the default then
that is of course fine but I wonder whether this might do ARMk more
harm than good in this case.

... 1023 event channels is going to be plenty enough for most of the use cases.


Julien Grall



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