RE: [PATCH v2 1/2] viridian: remove implicit limit of 64 VPs per partition

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
> Sent: 12 January 2021 04:17
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: iwj@xxxxxxxxxxxxxx; wl@xxxxxxx; andrew.cooper3@xxxxxxxxxx; 
> george.dunlap@xxxxxxxxxx;
> jbeulich@xxxxxxxx; julien@xxxxxxx; sstabellini@xxxxxxxxxx; 
> anthony.perard@xxxxxxxxxx; paul@xxxxxxx;
> roger.pau@xxxxxxxxxx; Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
> Subject: [PATCH v2 1/2] viridian: remove implicit limit of 64 VPs per 
> partition
> 
> TLFS 7.8.1 stipulates that "a virtual processor index must be less than
> the maximum number of virtual processors per partition" that "can be obtained
> through CPUID leaf 0x40000005". Furthermore, "Requirements for Implementing
> the Microsoft Hypervisor Interface" defines that starting from Windows Server
> 2012, which allowed more than 64 CPUs to be brought up, this leaf can now
> contain a value -1 basically assuming the hypervisor has no restriction while
> 0 (that we currently expose) means the default restriction is still present.
> 
> Along with the previous changes exposing ExProcessorMasks this allows a recent
> Windows VM with Viridian extension enabled to have more than 64 vCPUs without
> going into BSOD in some cases.
> 
> Since we didn't expose the leaf before and to keep CPUID data consistent for
> incoming streams from previous Xen versions - let's keep it behind an option.
> 
> Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>

Reviewed-by: Paul Durrant <paul@xxxxxxx>