Re: [PATCH v8 2/5] x86/tdx: Add TDX Guest event notify interrupt support

[Sathyanarayanan Kuppuswamy]

+ Jiewen

Jiewen, Can you please comment on the specification related queries?

On 6/20/22 8:44 AM, Sathyanarayanan Kuppuswamy wrote:
> Hi,
> 
> + Jun
> 
> On 6/20/22 5:33 AM, Kai Huang wrote:
>> On Wed, 2022-06-08 at 19:52 -0700, Kuppuswamy Sathyanarayanan wrote:
>>> Host-guest event notification via configured interrupt vector is useful
>>> in cases where a guest makes an asynchronous request and needs a
>>> callback from the host to indicate the completion or to let the host
>>> notify the guest about events like device removal. One usage example is,
>>> callback requirement of GetQuote asynchronous hypercall.
>>
>> Although this paragraph is from GHCI spec, IMHO it is not very helpful.  In
>> fact, I think this paragraph is not that right and should be removed from GHCI.
>> The reason is such event notification from VMM in cases like "device removal" is
>> too vague.  There's no _specification_ in GHCI around which "device removal"
>> should VMM inject such event.  For instance, I _think_ the Qemu enumerated ACPI-
>> based hotplug should continue to work in TD.
> 
> Yes. It just says that it *can* be used to signal a device removal. It is just
> an example for where it can be used. But I agree that such a use case is vague.
> If it makes it better, I am fine with removing it.
> 
> Copied from sec 3.5 TDG.VP.VMCALL<SetupEventNotifyInterrupt>:
> 
> "Example of an operation that can use the event notify is the host
> VMM signaling a device removal to the TD, in response to which a TD may
> unload a device driver."
> 
>>
>> That being said, if a TD has multiple devices, it cannot know whether the VMM
>> will inject the removal event via the vector set by SetupEventNotifyInterrupt. 
>> And for the same device in the same TD, different VMMs may use different way to
>> notify its removal.
> 
> As per current design, If it is used for device removal, I think all registered
> device drivers will get the notification and the individual device driver has
> to check whether it is applicable for them.
> 
> If the SetupEventNotifyInterrupt TDVMCALL specification is extended to specify
> the exact device or use case detail, then it can optimize the implementation.
> 
>>
>> It seems GetQuote is the only user of SetupEventNotifyInterrupt.  Maybe we
>> should just declare it is for GetQuote.
> 
> Ok.
> 
>>
>> Isaku, what do you think?  Does this make sense?
>>
>>>
>>> In TDX guest, SetupEventNotifyInterrupt hypercall can be used by the
>>> guest to specify which interrupt vector to use as an event-notify
>>> vector to the VMM. Details about the SetupEventNotifyInterrupt
>>> hypercall can be found in TDX Guest-Host Communication Interface
>>> (GHCI) Specification, sec 3.5 "VP.VMCALL<SetupEventNotifyInterrupt>".
>>> Add a tdx_hcall_set_notify_intr() helper function to implement the
>>> SetupEventNotifyInterrupt hypercall.
>>
>> As you also used "can" above, the GHCI only says the VMM _CAN_ inject the vector
>> set by SetupEventNotifyInterrupt, but not must (3.3 TDG.VP.VMCALL<GetQuote>). 
>> This means theoretically TD should implement pooling mode in case VMM doesn't
>> support injecting event via vector done by SetupEventNotifyInterrupt?
> 
> Yes. But GetQuote specification does not talk about the pooling mode
> use case as well. So I think it is just a wording confusion.
> 
>>
>> Perhaps we should update the GHCI spec to use must..
> 
> Ok.
> 
>>
>>>
>>> Reserve 0xec IRQ vector address for TDX guest to receive the event
>>> completion notification from VMM. Also add related IDT handler to
>>> process the notification event.
>>
>> Here lacks why we need to choose to reserve a system vector.  For instance, why
>> we cannot choose to use device IRQ way which only requires one vector on one
> 
> As you have explained below, as per current spec, it just expects a system
> vector.
> 
>> cpu.  As you can see reserving a system vector isn't ideal especially for
>> attestation as it is not a frequent operation.  It is wasteful of using IRQ
> 
> I agree that event notification is currently only used for attestation. But I
> think in future there could be other use cases for it. If the intention is just
> to use it for attestation, then we can just modify the GetQuote TDVMCALL to pass
> the vector address, and there is no need for new TDVMCALL. I think the intention
> here is to have generic method for VMM to notify TD about some events. I am not
> clear about the possible future use cases, so I cannot comment on frequency of
> its use.
> 
> Jun, any comments?
> 
> 
> 
>> resource especially on server systems with a lot of CPUs.
> 
> FWIW, this reservation is protected with CONFIG_INTEL_TDX_GUEST. So it will be
> reserved only for TDX use case.
> 
> 
>>
>> The reason is SetupEventNotifyInterrupt TDVMCALL only has one argument, which is
>> vector, but cannot specify which CPU that the VMM should inject the event to. 
>> The GHCI spec doesn't say which CPU the VMM should inject to (i.e. must inject
>> to the CPU on which SetupEventNotifyInterrupt is called), so we can only assume
>> VMM can inject to any CPU.
>>
>> Btw, x86 maintainers,
>>
>> I'd like to check with you to see whether we should improve the existing
>> SetupEventNotifyInterrupt so we can choose to use request_irq() style for
>> attestation.  Using request_irq() means we don't need to reserve a system
>> vector, but can allocate a vector dynamically when needed.
>>
>> Assuming we update SetupEventNotifyInterrupt to also allow TD to specify which
>> CPU (i.e. via APICID) to inject (along with the vector), my understanding is we
>> can use below way (idea only) to dynamically allocate a vector on one CPU when
>> attestation is needed:
>>
>>
>> 	int cpu, vector;
>> 	int irq;
>>
>> 	// request an IRQ, and prevent it from being migrated
>> 	irq = __irq_domain_alloc_irqs(x86_vector_domain, 0, 1, ...);
>> 	request_irq(irq, ...);
>> 	
>> 	// get vector, cpu from irq
>>
>> 	TDVMCALL<SetupEventNotifyInterrupt>(vector, 
>> 		apic->cpu_present_to_apidid(cpu));
>> 	
>> Is this reasonable? If yes, is it worth to do?
>>
> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer