Re: [PATCH v2 11/49] KVM: x86: Disallow KVM_CAP_X86_DISABLE_EXITS after vCPU creation

[Sean Christopherson]

On Thu, Jul 04, 2024, Maxim Levitsky wrote:
> On Fri, 2024-05-17 at 10:38 -0700, Sean Christopherson wrote:
> > Reject KVM_CAP_X86_DISABLE_EXITS if vCPUs have been created, as disabling
> > PAUSE/MWAIT/HLT exits after vCPUs have been created is broken and useless,
> > e.g. except for PAUSE on SVM, the relevant intercepts aren't updated after
> > vCPU creation.  vCPUs may also end up with an inconsistent configuration
> > if exits are disabled between creation of multiple vCPUs.
> 
> Hi,
> 
> I am not sure that PAUSE intercepts are updated either, I wasn't able to find a code
> that does this.
> 
> I agree with this change, but note that there was some talk on the mailing
> list to allow to selectively disable VM exits (e.g PAUSE, MWAIT, ...) only on
> some vCPUs, based on the claim that some vCPUs might run RT tasks, while some
> might be housekeeping.  I haven't followed those discussions closely.

This change is actually pulled from that series[*].  IIRC, v1 of that series
didn't close the VM-scoped hole, and the overall code was much more complex as
a result.

[*] https://lore.kernel.org/all/20230121020738.2973-2-kechenl@xxxxxxxxxx