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SubjectRe: [RFC PATCH v5 092/104] KVM: TDX: Handle TDX PV HLT hypercall
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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@intel.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.


Until that is fixed, KVM needs to do something like:

- every time a bit is set in PID.PIR, set tdx->buggy_hlt_workaround = 1

- every time TDG.VP.VMCALL<HLT> is received,
xchg(&tdx->buggy_hlt_workaround, 0) and return immediately to the guest
if it is 1.

Basically an internal version of PID.ON.

>>> + details.full = td_state_non_arch_read64(
>>> + to_tdx(vcpu), TD_VCPU_STATE_DETAILS_NON_ARCH);
>>
>> TDX documentation says "the meaning of the field may change with Intel TDX
>> module version", where is this field documented? I cannot find any "other
>> guest state" fields in the TDX documentation.
>
> IMO we should put a stake in the ground and refuse to accept code that consumes
> "non-architectural" state. It's all software, having non-architectural APIs is
> completely ridiculous.

Having them is fine, *using* them to work around undocumented bugs is
the ridiculous part.

You didn't answer the other question, which is "Where is R12 documented
for TDG.VP.VMCALL<Instruction.HLT>?" though... Should I be worried? :)


Paolo

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