Re: [PATCH] x86/kvm: refine condition for checking vCPU preempted

[Sean Christopherson <seanjc@google.com>]

On Wed, Mar 15, 2023, Li,Rongqing wrote:
> > Rather than have the guest rely on host KVM behavior clearing PV_UNHALT
> > when HLT is passed through), would it make sense to add something like
> > KVM_HINTS_HLT_PASSTHROUGH to more explicitly tell the guest that HLT isn't
> > intercepted?
> 
> KVM_HINTS_HLT_PASSTHROUGH is more obvious, but it need both kvm and guest support

Yeah, that's the downside.  But modifying KVM and/or the userspace VMM shouldn't
be difficult, i.e the only meaningful cost is the rollout of a new kernel/VMM.

On the other hand, establishing the heuristic that !PV_UNHALT == HLT_PASSTHROUGH
could have to subtle issues in the future.  It safe-ish in the context of this
patch as userspace is unlikely to set KVM_HINTS_REALTIME, hide PV_UNHALT, and _not_
passthrough HLT.  But without the REALTIME side of things, !PV_UNHALT == HLT_PASSTHROUGH
is much less likely to hold true.