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

[Claudio Carvalho <cclaudio@linux.ibm.com>]

On 8/14/19 7:46 AM, Michael Ellerman wrote:
> 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.

Dropped the CR save/restore in the next patchset version:

_GLOBAL(ucall_norets)
EXPORT_SYMBOL_GPL(ucall_norets)
        sc      2       /* Invoke the ultravisor */
        blr             /* Return r3 = status */


Thanks,
Claudio


>
> 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
>