Re: [Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/24/2015 12:10 PM, Juergen Gross wrote:
> If we can fiddle with the masks on boot, we could do it in a running
> system, too. Another advantage with not relying on cpuid. :-)

I am trying to catch up with this thread so I may have missed it, but I 
still don't understand why we don't want to rely on CPUID.
I think I saw Juergen said --- because it's HW-specific. But what's 
wrong with that? Hypervisor is building virtualized x86 (in this case) 
hardware and on such HW CPUID is the standard way of determining 
thread/core topology. Plus various ACPI tables and such.
And having a solution that doesn't address userspace (when there *is* a 
solution that can do it) doesn't seem like the best approach. Yes, it 
still won't cover userspace for PV guests but neither will the kernel patch.
As far as licensing is concerned --- are we sure this can't also be 
addressed by CPUID? BTW, if I was asked about who is most concerned 
about licensing my first answer would be --- databases. I.e. userspace.
(Also, I don't know whether this is still true but in the past APICID 
format was also used for topology discovery. Just to make things a bit 
more interesting ;-))

-boris



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