On Thu, May 09, 2024, Isaku Yamahata wrote: > On Fri, May 10, 2024 at 11:19:44AM +1200, Kai Huang <kai.huang@xxxxxxxxx> 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.
