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

[Suzuki K Poulose]

Hi,

A few cents from the Realm support point of view.

On 05/02/2023 10:12, Marc Zyngier wrote:
> On Fri, 03 Feb 2023 13:50:40 +0000,
> James Morse <james.morse at arm.com> wrote:
>>
>> From: Jean-Philippe Brucker <jean-philippe at linaro.org>
>>
>> When capability KVM_CAP_ARM_HVC_TO_USER is available, userspace can
>> request to handle all hypercalls that aren't handled by KVM. With the
>> help of another capability, this will allow userspace to handle PSCI
>> calls.
>>
>> Suggested-by: James Morse <james.morse at arm.com>
>> Signed-off-by: Jean-Philippe Brucker <jean-philippe at linaro.org>
>> Signed-off-by: James Morse <james.morse at arm.com>
>>
>> ---
>>
> 
> On top of Oliver's ask not to make this a blanket "steal everything",
> but instead to have an actual request for ranges of forwarded
> hypercalls:
> 
>> Notes on this implementation:
>>
>> * A similar mechanism was proposed for SDEI some time ago [1]. This RFC
>>    generalizes the idea to all hypercalls, since that was suggested on
>>    the list [2, 3].
>>
>> * We're reusing kvm_run.hypercall. I copied x0-x5 into
>>    kvm_run.hypercall.args[] to help userspace but I'm tempted to remove
>>    this, because:
>>    - Most user handlers will need to write results back into the
>>      registers (x0-x3 for SMCCC), so if we keep this shortcut we should
>>      go all the way and read them back on return to kernel.
>>    - QEMU doesn't care about this shortcut, it pulls all vcpu regs before
>>      handling the call.

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.

Thanks
Suzuki