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

[Michael Ellerman <mpe@ellerman.id.au>]

Claudio Carvalho <cclaudio@linux.ibm.com> writes:
> diff --git a/arch/powerpc/kernel/ucall.S b/arch/powerpc/kernel/ucall.S
> new file mode 100644
> index 000000000000..de9133e45d21
> --- /dev/null
> +++ b/arch/powerpc/kernel/ucall.S
> @@ -0,0 +1,20 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Generic code to perform an ultravisor call.
> + *
> + * Copyright 2019, IBM Corporation.
> + *
> + */
> +#include <asm/ppc_asm.h>
> +#include <asm/export.h>
> +
> +_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).

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.

cheers