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Date: Fri, 10 May 2024 07:04:54 -0700
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Subject: Re: [PATCH v19 037/130] KVM: TDX: Make KVM_CAP_MAX_VCPUS backend specific
From: Sean Christopherson <seanjc@google.com>
To: Isaku Yamahata <isaku.yamahata@intel.com>
Cc: Kai Huang <kai.huang@intel.com>, "kvm@vger.kernel.org" <kvm@vger.kernel.org>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, 
	"isaku.yamahata@gmail.com" <isaku.yamahata@gmail.com>, Paolo Bonzini <pbonzini@redhat.com>, 
	Erdem Aktas <erdemaktas@google.com>, Sagi Shahar <sagis@google.com>, Bo2 Chen <chen.bo@intel.com>, 
	Hang Yuan <hang.yuan@intel.com>, Tina Zhang <tina.zhang@intel.com>, 
	isaku.yamahata@linux.intel.com
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On Thu, May 09, 2024, Isaku Yamahata wrote:
> On Fri, May 10, 2024 at 11:19:44AM +1200, Kai Huang <kai.huang@intel.com> wrote:
> > On 10/05/2024 10:52 am, Sean Christopherson wrote:
> > > On Fri, May 10, 2024, Kai Huang wrote:
> > > > On 10/05/2024 4:35 am, Sean Christopherson wrote:
> > > > > KVM x86 limits KVM_MAX_VCPUS to 4096:
> > > > > 
> > > > >     config KVM_MAX_NR_VCPUS
> > > > > 	int "Maximum number of vCPUs per KVM guest"
> > > > > 	depends on KVM
> > > > > 	range 1024 4096
> > > > > 	default 4096 if MAXSMP
> > > > > 	default 1024
> > > > > 	help
> > > > > 
> > > > > whereas the limitation from TDX is apprarently simply due to TD_PARAMS taking
> > > > > a 16-bit unsigned value:
> > > > > 
> > > > >     #define TDX_MAX_VCPUS  (~(u16)0)
> > > > > 
> > > > > i.e. it will likely be _years_ before TDX's limitation matters, if it ever does.
> > > > > And _if_ it becomes a problem, we don't necessarily need to have a different
> > > > > _runtime_ limit for TDX, e.g. TDX support could be conditioned on KVM_MAX_NR_VCPUS
> > > > > being <= 64k.
> > > > 
> > > > Actually later versions of TDX module (starting from 1.5 AFAICT), the module
> > > > has a metadata field to report the maximum vCPUs that the module can support
> > > > for all TDX guests.
> > > 
> > > My quick glance at the 1.5 source shows that the limit is still effectively
> > > 0xffff, so again, who cares?  Assert on 0xffff compile time, and on the reported
> > > max at runtime and simply refuse to use a TDX module that has dropped the minimum
> > > below 0xffff.
> > 
> > I need to double check why this metadata field was added.  My concern is in
> > future module versions they may just low down the value.
> 
> TD partitioning would reduce it much.

That's still not a reason to plumb in what is effectively dead code.  Either
partitioning is opt-in, at which I suspect KVM will need yet more uAPI to express
the limitations to userspace, or the TDX-module is potentially breaking existing
use cases.