Re: [PATCH v2 5/8] KVM: TDX: Handle TDG.VP.VMCALL<MapGPA>

[Binbin Wu]

On 2/19/2025 8:29 AM, Sean Christopherson wrote:
> On Mon, Feb 17, 2025, Binbin Wu wrote:
> > On 2/13/2025 11:17 PM, Sean Christopherson wrote:
> > > On Thu, Feb 13, 2025, Binbin Wu wrote:
> > > > On 2/13/2025 11:23 AM, Binbin Wu wrote:
> > > > > On 2/13/2025 2:56 AM, Sean Christopherson wrote:
> > > > > > On Wed, Feb 12, 2025, Binbin Wu wrote:
> > > > > > > On 2/12/2025 8:46 AM, Sean Christopherson wrote:
> > > > > > > > I am completely comfortable saying that KVM doesn't care about STI/SS shadows
> > > > > > > > outside of the HALTED case, and so unless I'm missing something, I think it makes
> > > > > > > > sense for tdx_protected_apic_has_interrupt() to not check RVI outside of the HALTED
> > > > > > > > case, because it's impossible to know if the interrupt is actually unmasked, and
> > > > > > > > statistically it's far, far more likely that it _is_ masked.
> > > > > > > OK. Will update tdx_protected_apic_has_interrupt() in "TDX interrupts" part.
> > > > > > > And use kvm_vcpu_has_events() to replace the open code in this patch.
> > > > > > Something to keep an eye on: kvm_vcpu_has_events() returns true if pv_unhalted
> > > > > > is set, and pv_unhalted is only cleared on transitions KVM_MP_STATE_RUNNABLE.
> > > > > > If the guest initiates a spurious wakeup, pv_unhalted could be left set in
> > > > > > perpetuity.
> > > > > Oh, yes.
> > > > > KVM_HC_KICK_CPU is allowed in TDX guests.
> > > And a clever guest can send a REMRD IPI.
> > >
> > > > > The change below looks good to me.
> > > > >
> > > > > One minor issue is when guest initiates a spurious wakeup, pv_unhalted is
> > > > > left set, then later when the guest want to halt the vcpu, in
> > > > > __kvm_emulate_halt(), since pv_unhalted is still set and the state will not
> > > > > transit to KVM_MP_STATE_HALTED.
> > > > > But I guess it's guests' responsibility to not initiate spurious wakeup,
> > > > > guests need to bear the fact that HLT could fail due to a previous
> > > > > spurious wakeup?
> > > > Just found a patch set for fixing the issue.
> > > FWIW, Jim's series doesn't address spurious wakeups per se, it just ensures
> > > pv_unhalted is cleared when transitioning to RUNNING.  If the vCPU is already
> > > RUNNING, __apic_accept_irq() will set pv_unhalted and nothing will clear it
> > > until the next transition to RUNNING (which implies at least an attempted
> > > transition away from RUNNING).
> > Indeed.
> >
> > I am wondering why KVM doesn't clear pv_unhalted before the vcpu entering guest?
> > Is the additional memory access a concern or is there some other reason?
> Not clearing pv_unhalted when entering the guest is necessary to avoid races.
> Stating the obvious, the guest must set up all of its lock tracking before executing
> HLT, which means that the soon-to-be-blocking vCPU is eligible for wakeup *before*
> it executes HLT.  If an asynchronous exit happens on the vCPU at just the right
> time, KVM could clear pv_unhalted before the vCPU executes HLT.
Got it.
Thanks for the explanation.