On Wed, 08 Feb 2023 08:40:09 +0000,
Marc Zyngier [off-list ref] wrote:

On Tue, 07 Feb 2023 17:50:58 +0000,
James Morse [off-list ref] wrote:

quoted

Hi Marc,

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.

quoted

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.
  - SMCCC uses x0-x16 for parameters.
  x0 does contain the SMCCC function ID and may be useful for fast
  dispatch, we could keep that plus the immediate number.

* Add a flag in the kvm_run.hypercall telling whether this is HVC or
  SMC?  Can be added later in those bottom longmode and pad fields.

quoted

We definitely need this. A nested hypervisor can (and does) use SMCs
as the conduit.

Christoffer's comments last time round on this was that EL2 guests
get SMC with this, and EL1 guests get HVC. The VMM could never get
both...

I agree with the first half of the statement (EL2 guest using SMC),
but limiting EL1 guests to HVC is annoying. On systems that have a
secure side, it would make sense to be able to route the guest's SMC
calls to userspace and allow it to emulate/proxy/deny such calls.

You also want to look at the TRNG firmware spec (aka DEN0098), which
explicitly calls out for the use of SMC when EL2 and EL3 are
implemented (see 1.5 "Invocation considerations").

Is it mad? Yes. But madness seems to be the direction of travel these
days.

	M.

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

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel