[PATCH v5 2/7] powerpc/kernel: Add ucall_norets() ultravisor call handler

[Claudio Carvalho]

On 8/14/19 3:34 PM, Segher Boessenkool wrote:
> On Wed, Aug 14, 2019 at 08:46:15PM +1000, Michael Ellerman wrote:
>> Claudio Carvalho <cclaudio at linux.ibm.com> writes:
>>> +_GLOBAL(ucall_norets)
>>> +EXPORT_SYMBOL_GPL(ucall_norets)
>>> +	mfcr	r0
>>> +	stw	r0,8(r1)
>>> +
>>> +	sc	2		/* Invoke the ultravisor */
>>> +
>>> +	lwz	r0,8(r1)
>>> +	mtcrf	0xff,r0
>>> +	blr			/* Return r3 = status */
>> Paulus points that we shouldn't need to save CR here. Our caller will
>> have already saved it if it needed to, and we don't use CR in this
>> function so we don't need to save it.
>>
>> That's assuming the Ultravisor follows the hcall ABI in which CR2-4 are
>> non-volatile (PAPR § 14.5.3).
> And assuming the ultravisor already clears (or sets, or whatever) all CR
> fields it does not want to leak the contents of (which it also should,
> of course).

Thanks Segher. We are working on that in the ultravisor source code.

Claudio.


>
>> I know plpar_hcall_norets() does save CR, but it shouldn't need to, that
>> seems to be historical. aka. no one knows why it does it but it always
>> has.
>
> Segher
>