Re: [Xen-devel] [RFC Patch v4 8/8] x86/hvm: bump the maximum number of vcpus to 512

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> Chao Gao <chao.gao@xxxxxxxxx> 03/01/18 7:34 AM >>>
>On Mon, Feb 26, 2018 at 09:10:33AM -0700, Jan Beulich wrote:
>>Again - here we're talking about implementation limits, not
>>bottlenecks. So in this context all I'm interested in is whether
>>(and if so which) implementation limit remains. If an (almost)
>>arbitrary number is fine, perhaps we'll want to have a Kconfig
>>option.
>
>Do you think that struct hvm_info_table would be a implementation
>limits? To contain this struct in a single page, the HVM_MAX_VCPUS
>should be smaller than a value, like (PAGE_SIZE * 8). Supposing
>it is the only implementation limit, I don't think it is reasonable
>to set HVM_MAX_VCPUS to that value, because we don't have hardwares to
>perform tests, even Xeon-phi isn't capable. This value can be bumped
>when some methods verify a guest can work with more vcpus. Now I
>prefer 288 over 512 and some values else.

Whether going beyond PAGE_SIZE with the structure size is a valid item
to think about, but I don't think there's any implied limit from that. But -
did you read my and George's subsequent reply at all? You continue to
mix up supported (because of being able to test) limits with implementation
ones. Even Jürgen's suggestion to take NR_CPUS as the limit is not very
reasonable - PV guests have an implementation limit of (iirc) 8192. Once
again - if there's no sensible upper limit imposed by the implementation,
consider introducing a Kconfig option to pick the limit.

Jan


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