linux-kernel - Re: [RFC PATCH v5 092/104] KVM: TDX: Handle TDX PV HLT hypercall

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Date:   Thu, 7 Apr 2022 16:08:25 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
        Jim Mattson <jmattson@...gle.com>, erdemaktas@...gle.com,
        Connor Kuehl <ckuehl@...hat.com>
Subject: Re: [RFC PATCH v5 092/104] KVM: TDX: Handle TDX PV HLT hypercall

On Thu, Apr 07, 2022, Paolo Bonzini wrote:
> On 4/7/22 17:02, Sean Christopherson wrote:
> > On Thu, Apr 07, 2022, Paolo Bonzini wrote:
> > > On 3/4/22 20:49, isaku.yamahata@...el.com wrote:
> > > > +	bool interrupt_disabled = tdvmcall_p1_read(vcpu);
> > > 
> > > Where is R12 documented for TDG.VP.VMCALL<Instruction.HLT>?
> > > 
> > > > +		 * Virtual interrupt can arrive after TDG.VM.VMCALL<HLT> during
> > > > +		 * the TDX module executing.  On the other hand, KVM doesn't
> > > > +		 * know if vcpu was executing in the guest TD or the TDX module.
> > > 
> > > I don't understand this; why isn't it enough to check PI.ON or something
> > > like that as part of HLT emulation?
> > 
> > Ooh, I think I remember what this is.  This is for the case where the virtual
> > interrupt is recognized, i.e. set in vmcs.RVI, between the STI and "HLT".  KVM
> > doesn't have access to RVI and the interrupt is no longer in the PID (because it
> > was "recognized".  It doesn't get delivered in the guest because the TDCALL
> > completes before interrupts are enabled.
> > 
> > I lobbied to get this fixed in the TDX module by immediately resuming the guest
> > in this case, but obviously that was unsuccessful.
> 
> So the TDX module sets RVI while in an STI interrupt shadow.  So far so
> good.  Then:
> 
> - it receives the HLT TDCALL from the guest.  The interrupt shadow at this
> point is gone.
> 
> - it knows that there is an interrupt that can be delivered (RVI > PPR &&
> EFLAGS.IF=1, the other conditions of 29.2.2 don't matter)
> 
> - it forwards the HLT TDCALL nevertheless, to a clueless hypervisor that has
> no way to glean either RVI or PPR?
> 
> It's absurd that this be treated as anything but a bug.

That's what I said!  :-)