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

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Message-ID: <Z64M_r64CCWxSD5_@google.com>
Date: Thu, 13 Feb 2025 07:17:18 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
Cc: Chao Gao <chao.gao@...el.com>, Yan Zhao <yan.y.zhao@...el.com>, pbonzini@...hat.com, 
	kvm@...r.kernel.org, rick.p.edgecombe@...el.com, kai.huang@...el.com, 
	adrian.hunter@...el.com, reinette.chatre@...el.com, xiaoyao.li@...el.com, 
	tony.lindgren@...el.com, isaku.yamahata@...el.com, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/8] KVM: TDX: Handle TDG.VP.VMCALL<MapGPA>

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).