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 [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.

Thanks
Suzuki

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