Re: [PATCH v19 037/130] KVM: TDX: Make KVM_CAP_MAX_VCPUS backend specific

[Huang, Kai]

On Tue, 2024-06-04 at 10:48 +0000, Huang, Kai wrote:
> On Thu, 2024-05-30 at 16:12 -0700, Sean Christopherson wrote:
> > On Thu, May 30, 2024, Kai Huang wrote:
> > > On Wed, 2024-05-29 at 16:15 -0700, Sean Christopherson wrote:
> > > > In the unlikely event there is a legitimate reason for max_vcpus_per_td being
> > > > less than KVM's minimum, then we can update KVM's minimum as needed.  But AFAICT,
> > > > that's purely theoretical at this point, i.e. this is all much ado about nothing.
> > > 
> > > I am afraid we already have a legitimate case: TD partitioning.  Isaku
> > > told me the 'max_vcpus_per_td' is lowed to 512 for the modules with TD
> > > partitioning supported.  And again this is static, i.e., doesn't require
> > > TD partitioning to be opt-in to low to 512.
> > 
> > So what's Intel's plan for use cases that creates TDs with >512 vCPUs?
> 
> I checked with TDX module guys.  Turns out the 'max_vcpus_per_td' wasn't
> introduced because of TD partitioning, and they are not actually related.
> 
> They introduced this to support "topology virtualization", which requires
> a table to record the X2APIC IDs for all vcpus for each TD.  In practice,
> given a TDX module, the 'max_vcpus_per_td', a.k.a, the X2APIC ID table
> size reflects the physical logical cpus that *ALL* platforms that the
> module supports can possibly have.
> 
> The reason of this design is TDX guys don't believe there's sense in
> supporting the case where the 'max_vcpus' for one single TD needs to
> exceed the physical logical cpus.
> 
> So in short:
> 
> - The "max_vcpus_per_td" can be different depending on module versions. In
> practice it reflects the maximum physical logical cpus that all the
> platforms (that the module supports) can possibly have.
> 
> - Before CSPs deploy/migrate TD on a TDX machine, they must be aware of
> the "max_vcpus_per_td" the module supports, and only deploy/migrate TD to
> it when it can support.
> 
> - For TDX 1.5.xx modules, the value is 576 (the previous number 512 isn't
> correct); For TDX 2.0.xx modules, the value is larger (>1000).  For future
> module versions, it could have a smaller number, depending on what
> platforms that module needs to support.  Also, if TDX ever gets supported
> on client platforms, we can image the number could be much smaller due to
> the "vcpus per td no need to exceed physical logical cpus".
> 
> We may ask them to support the case where 'max_vcpus' for single TD
> exceeds the physical logical cpus, or at least not to low down the value
> any further for future modules (> 2.0.xx modules).  We may also ask them
> to give promise to not low the number to below some certain value for any
> future modules.  But I am not sure there's any concrete reason to do so?
> 
> What's your thinking?
> 

Hi Sean,

Sorry to ping, but do you have any comments on this?

Is there anything we should make TDX module guys do?