Re: [Xen-devel] [PATCH 03/10] x86/cpuid: Handle leaf 0x1 in guest_cpuid()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 21/02/17 17:20, Jan Beulich wrote:
>
>>>> The final 8 bits are the initial legacy APIC ID.  For HVM guests, this was
>>>> overridden to vcpu_id * 2.  The same logic is now applied to PV guests, so
>>>> guests don't observe a constant number on all vcpus via their emulated or
>>>> faulted view.
>>> They won't be the same everywhere, but every 128th CPU will
>>> share values. I'm therefore not sure it wouldn't be better to hand
>>> out zero or all ones here.
>> There is no case where 128 cpus work sensibly under Xen ATM.
> For HVM you mean. I'm sure I've seen > 128 vCPU PV guests
> (namely Dom0-s).

You can physically create PV domains with up to 8192 vcpus.  I tried
this once.

The NMI watchdog (even set to 10s) is unforgiving of some the
for_each_vcpu() loops during domain destruction.

I can also still create workloads in a 64vcpu HVM guest which will cause
a 5 second watchdog timeout, which is why XenServers upper supported
vcpu limit is still 32.

~Andrew

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