Re: [PATCH v13 018/113] KVM: x86, tdx: Make KVM_CAP_MAX_VCPUS backend specific

[Isaku Yamahata]

On Sat, Mar 25, 2023 at 08:13:26PM +0200,
Zhi Wang <zhi.wang.linux@xxxxxxxxx> wrote:

> On Sun, 12 Mar 2023 10:55:42 -0700
> isaku.yamahata@xxxxxxxxx wrote:
> 
> > From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> > 
> > TDX has its own limitation on the maximum number of vcpus that the guest
> > can accommodate.  Allow x86 kvm backend to implement its own KVM_ENABLE_CAP
> > handler and implement TDX backend for KVM_CAP_MAX_VCPUS.  user space VMM,
> > e.g. qemu, can specify its value instead of KVM_MAX_VCPUS.
> > 
> 
> I think enabling the cap here is actually "configuring the cap". KVM_CAP_MAX
> _VCPUS is actually always enabled whether userspace enables it or not. It
> would be nice to configure of the max VCPUS in kvm_arch_vm_ioctl() where
> routines of configuring a VM should belong. E.g. KVM_SET_MAX_VCPUS.

I'm not sure I understand your point.  Although KVM_ENABLE_CAP sounds like
only on/off feature, but it isn't. It's also used to set parameters. For
example, KVM_CAP_MAX_VCPU_ID.

KVM_SET_XXX is for run time feature. But the maxium number of vcpus is not
runtime changable. Set it at vm creation.
-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxx>