On Mon, 06 Feb 2023 10:10:41 +0000,
Suzuki K Poulose [off-list ref] wrote:

Hi,

A few cents from the Realm support point of view.

On 05/02/2023 10:12, Marc Zyngier wrote:

quoted

On Fri, 03 Feb 2023 13:50:40 +0000,
James Morse [off-list ref] wrote:

quoted

From: Jean-Philippe Brucker <redacted>

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@arm.com>
Signed-off-by: Jean-Philippe Brucker <redacted>
Signed-off-by: James Morse <james.morse@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:

quoted

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.

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.

Of course, it requires that the hypervisor (the RMM in your case)
knows about the semantics of the hypercall, but that's obviously
already a requirement (or you wouldn't be able to use PSCI at all).

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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