Re: [PATCH v6 7/7] powerpc/kvm: Use UV_RETURN ucall to return to ultravisor

On Thu, Aug 22, 2019 at 12:48:38AM -0300, Claudio Carvalho wrote:

From: Sukadev Bhattiprolu <redacted>

When an SVM makes an hypercall or incurs some other exception, the
Ultravisor usually forwards (a.k.a. reflects) the exceptions to the
Hypervisor. After processing the exception, Hypervisor uses the
UV_RETURN ultracall to return control back to the SVM.

The expected register state on entry to this ultracall is:

* Non-volatile registers are restored to their original values.
* If returning from an hypercall, register R0 contains the return value
  (unlike other ultracalls) and, registers R4 through R12 contain any
  output values of the hypercall.
* R3 contains the ultracall number, i.e UV_RETURN.
* If returning with a synthesized interrupt, R2 contains the
  synthesized interrupt number.

This isn't accurate: R2 contains the value that should end up in SRR1
when we are back in the secure guest.  HSRR0 and HSRR1 contain the
instruction pointer and MSR that the guest should run with.  They may
be different from the instruction pointer and MSR that the guest vCPU
last had, if the hypervisor has synthesized an interrupt for the
guest.  The ultravisor needs to detect this case and respond
appropriately.

Thanks to input from Paul Mackerras, Ram Pai and Mike Anderson.

Signed-off-by: Sukadev Bhattiprolu <redacted>
Signed-off-by: Claudio Carvalho <redacted>

Apart from that comment on the patch description -

Acked-by: Paul Mackerras <redacted>