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

On 8/14/19 3:34 PM, Segher Boessenkool wrote:

On Wed, Aug 14, 2019 at 08:46:15PM +1000, Michael Ellerman wrote:

quoted

Claudio Carvalho [off-list ref] writes:

quoted

+_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.

quoted

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