Re: [RFC PATCH 29/32] KVM: arm64: Pass hypercalls to userspace

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 07/02/2023 11:23, Marc Zyngier wrote:
> On Tue, 07 Feb 2023 09:41:54 +0000,
> Suzuki K Poulose <suzuki.poulose@arm.com> wrote:
>>
>> Hi Marc,
>>
>> On 06/02/2023 12:31, Marc Zyngier wrote:
>>> On Mon, 06 Feb 2023 10:10:41 +0000,
>>> Suzuki K Poulose <suzuki.poulose@arm.com> wrote:
>>>>
>>>> This may not be always possible, e.g., for Realms. GET_ONE_REG is
>>>> not supported. So using an explicit passing down of the args is
>>>> preferrable.
>>>
>>> What is the blocker for CCA to use GET_ONE_REG? The value obviously
>>> exists and is made available to the host. pKVM is perfectly able to
>>> use GET_ONE_REG and gets a bunch of zeroes for things that the
>>> hypervisor has decided to hide from the host.
>>>
>>
>> It is not impossible. On a "HOST CALL" (explicit calls to the Host
>> from Realm), the GPRs are made available to the host and can be
>> stashed into the vcpu reg state and the request can be
>> serviced. However, it is a bit odd, to make this exception - "the
>> GET_ONE_REG is valid now", while in almost all other cases it is
>> invalid (exception of MMIO).
> 
> But that's an RMM decision. If the RMM decides to forward the
> hypercall to the host (irrespective of the potential forwarding to
> userspace), it makes the GPRs available.
> 
> If the hypercall is forwarded to userspace, then the host is
> responsible to check with the RMM that it will be willing to provide
> the required information (passed as GPRs or not).

Just to be clear, on a hypercall, all the arguments are provided to
the host. And it is always possible for the host to sync the vcpu
GPR state with those arguments and make them available via the 
GET_ONE_REG call.

> 
>> Of course we could always return what is stashed in the vcpu state,
>> which is may be invalid/ 0. But given the construct of "host doesn't
>> have access to the register state", it may be a good idea to say,
>> request always fails, to indicate that the Host is probably doing
>> something wrong, than silently passing on incorrect information.
> 
> I disagree. Either you fail at the delegation point, or you don't. On
> getting a hypercall exit to userspace, you are guaranteed that the
> GPRs are valid.

This is possible, as I mentioned below, the question is bug vs feature.

> 
>>> Of course, it requires that the hypervisor (the RMM in your case)
>>> knows about the semantics of the hypercall, but that's obviously
>>
>> RMM doesn't care about the semantics of hypercall, other than
>> considering it just like an SMCCC compliant call. The hypercall
>> arguments/results are passed down/up by the Realm in a separate
>> structure.
> 
> That's because the RMM doesn't use registers to pass the data. But at
> the end of the day, this is the same thing. The host gets the data
> from the RMM, stashes it in the GPRs, and exit to userspace.

True.

> 
> The important thing here is that GET_ONE_REG is valid in the context
> where it matters. If the VMM tries to use it outside of the context of
> a hypercall, it gets junk. It's not a bug, it's a feature.

This is what I was concerned about.  As long as this "For any exit
other than hypercall (at least for now), you get junk values when using
GET_ONE_REG for confidential guests" is an acceptable feature, that 
should be alright.

Thanks
Suzuki

> 
> Thanks,
> 
> 	M.
>