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

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Paul Durrant <xadimgnik@xxxxxxxxx>
> Sent: 11 January 2021 09:10
> To: 'Jan Beulich' <jbeulich@xxxxxxxx>
> Cc: wl@xxxxxxx; iwj@xxxxxxxxxxxxxx; anthony.perard@xxxxxxxxxx; 
> andrew.cooper3@xxxxxxxxxx;
> george.dunlap@xxxxxxxxxx; julien@xxxxxxx; sstabellini@xxxxxxxxxx; 
> roger.pau@xxxxxxxxxx; xen-
> devel@xxxxxxxxxxxxxxxxxxxx; 'Igor Druzhinin' <igor.druzhinin@xxxxxxxxxx>
> Subject: RE: [PATCH 1/2] viridian: remove implicit limit of 64 VPs per 
> partition
> 
> > -----Original Message-----
> > From: Jan Beulich <jbeulich@xxxxxxxx>
> > Sent: 11 January 2021 09:00
> > To: paul@xxxxxxx
> > Cc: wl@xxxxxxx; iwj@xxxxxxxxxxxxxx; anthony.perard@xxxxxxxxxx; 
> > andrew.cooper3@xxxxxxxxxx;
> > george.dunlap@xxxxxxxxxx; julien@xxxxxxx; sstabellini@xxxxxxxxxx; 
> > roger.pau@xxxxxxxxxx; xen-
> > devel@xxxxxxxxxxxxxxxxxxxx; 'Igor Druzhinin' <igor.druzhinin@xxxxxxxxxx>
> > Subject: Re: [PATCH 1/2] viridian: remove implicit limit of 64 VPs per 
> > partition
> >
> > On 11.01.2021 09:45, Paul Durrant wrote:
> > >> -----Original Message-----
> > >> From: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
> > >> Sent: 09 January 2021 00:56
> > >> To: paul@xxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx
> > >> Cc: wl@xxxxxxx; iwj@xxxxxxxxxxxxxx; anthony.perard@xxxxxxxxxx; 
> > >> andrew.cooper3@xxxxxxxxxx;
> > >> george.dunlap@xxxxxxxxxx; jbeulich@xxxxxxxx; julien@xxxxxxx; 
> > >> sstabellini@xxxxxxxxxx;
> > >> roger.pau@xxxxxxxxxx
> > >> Subject: Re: [PATCH 1/2] viridian: remove implicit limit of 64 VPs per 
> > >> partition
> > >>
> > >> On 08/01/2021 08:32, Paul Durrant wrote:
> > >>>> -----Original Message-----
> > >>>> From: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
> > >>>> Sent: 08 January 2021 00:47
> > >>>> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> > >>>> Cc: paul@xxxxxxx; wl@xxxxxxx; iwj@xxxxxxxxxxxxxx; 
> > >>>> anthony.perard@xxxxxxxxxx;
> > >>>> andrew.cooper3@xxxxxxxxxx; george.dunlap@xxxxxxxxxx; 
> > >>>> jbeulich@xxxxxxxx; julien@xxxxxxx;
> > >>>> sstabellini@xxxxxxxxxx; roger.pau@xxxxxxxxxx; Igor Druzhinin 
> > >>>> <igor.druzhinin@xxxxxxxxxx>
> > >>>> Subject: [PATCH 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 previous changes exposing ExProcessorMasks this allows a 
> > >>>> recent
> > >>>> Windows VM with Viridian extension enabled to have more than 64 vCPUs 
> > >>>> without
> > >>>> going into immediate BSOD.
> > >>>>
> > >>>
> > >>> This is very odd as I was happily testing with a 128 vCPU VM once 
> > >>> ExProcessorMasks was
> > >> implemented... no need for the extra leaf.
> > >>> The documentation for 0x40000005 states " Describes the scale limits 
> > >>> supported in the current
> > >> hypervisor implementation. If any
> > >>> value is zero, the hypervisor does not expose the corresponding 
> > >>> information". It does not say
> (in
> > >> section 7.8.1 or elsewhere AFAICT)
> > >>> what implications that has for VP_INDEX.
> > >>>
> > >>> In " Requirements for Implementing the Microsoft Hypervisor Interface" 
> > >>> I don't see anything
> saying
> > >> what the semantics of not
> > >>> implementing leaf 0x40000005 are, only that if implementing it then -1 
> > >>> must be used to break the
> > 64
> > >> VP limit. It also says that
> > >>> reporting -1 in 0x40000005 means that 40000004.EAX bits 1 and 2 *must* 
> > >>> be clear, which is
> clearly
> > >> not true if ExProcessorMasks is
> > >>> enabled. That document is dated June 13th 2012 so I think it should be 
> > >>> taken with a pinch of
> salt.
> > >>>
> > >>> Have you actually observed a BSOD with >64 vCPUs when ExProcessorMasks 
> > >>> is enabled? If so, which
> > >> version of Windows? I'd like to get
> > >>> a repro myself.
> > >>
> > >> I return with more testing that confirm both my and your results.
> > >>
> > >> 1) with ExProcessorMask and 66 vCPUs assigned both current WS19 build and
> > >> pre-release 20270 of vNext boot correctly with no changes
> > >>
> > >> and that would be fine but the existing documentation for 
> > >> ex_processor_masks
> > >> states that:
> > >> "Hence this enlightenment must be specified for guests with more
> > >> than 64 vCPUs if B<hcall_remote_tlb_flush> and/or B<hcall_ipi> are also
> > >> specified."
> > >>
> > >> You then would expect 64+ vCPU VM to boot without ex_processors_mask,
> > >> hcall_remote_tlb_flush and hcall_ipi.
> > >
> > > Indeed.
> > >
> > >>
> > >> So,
> > >> 2) without ExProcessorMaks and 66 vCPUs assigned and with 
> > >> hcall_remote_tlb_flush
> > >> and hcall_ipi disabled: WS19 BSODs and vNext fails to initialize 
> > >> secondary CPUs
> > >>
> > >
> > > This is not what I recall from testing but I can confirm I see the same 
> > > now.
> > >
> > >> After applying my change,
> > >> 3) without ExProcessorMaks and 66 vCPUs assigned and with 
> > >> hcall_remote_tlb_flush
> > >> and hcall_ipi disabled WS19 and vNext boot correctly
> > >>
> > >> So we either need to change documentation and require ExProcessorMasks 
> > >> for all
> > >> VMs with 64+ vCPUs or go with my change.
> > >>
> > >
> > > I think we go with your change. I guess we can conclude then that the 
> > > separate viridian flag as
> part
> > of the base set is the right way to go (to stop the leaf magically 
> > appearing on migrate of existing
> > guests) and leave ex_processor_masks (and the documentation) as-is.
> > >
> > > You can add my R-b to the patch.
> >
> > That's the unchanged patch then, including the libxl change that
> > I had asked about and that I have to admit I don't fully follow
> > Igor's responses? I'm hesitant to give an ack for that aspect of
> > the change, yet I suppose the libxl maintainers will defer to
> > x86 ones there. Alternatively Andrew or Roger could of course
> > ack this ...
> >
> 
> I don't think we really need specific control in xl.cfg as this is a fix for 
> some poorly documented
> semantics in the spec. The flag simply prevents the leaf magically appearing 
> on migrate and I think
> that's enough.

... although adding an option in xl/libxl isn't that much work, I suppose.

Igor, would you be ok plumbing it through?

  Paul

> 
>   Paul
> 
> > Jan