Re: [kvm-unit-tests PATCH v8 03/12] s390x: saving regs for interrupts

[Thomas Huth <thuth@redhat.com>]

On 08/06/2020 10.12, Pierre Morel wrote:
> If we use multiple source of interrupts, for example, using SCLP
> console to print information while using I/O interrupts, we need
> to have a re-entrant register saving interruption handling.
> 
> Instead of saving at a static memory address, let's save the base
> registers, the floating point registers and the floating point
> control register on the stack in case of I/O interrupts
> 
> Note that we keep the static register saving to recover from the
> RESET tests.
> 
> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> Acked-by: Janosch Frank <frankja@linux.ibm.com>
> ---
>  s390x/cstart64.S | 41 +++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 39 insertions(+), 2 deletions(-)
> 
> diff --git a/s390x/cstart64.S b/s390x/cstart64.S
> index b50c42c..a9d8223 100644
> --- a/s390x/cstart64.S
> +++ b/s390x/cstart64.S
> @@ -119,6 +119,43 @@ memsetxc:
>  	lmg	%r0, %r15, GEN_LC_SW_INT_GRS
>  	.endm
>  
> +/* Save registers on the stack (r15), so we can have stacked interrupts. */
> +	.macro SAVE_REGS_STACK
> +	/* Allocate a stack frame for 15 general registers */
> +	slgfi   %r15, 15 * 8
> +	/* Store registers r0 to r14 on the stack */
> +	stmg    %r0, %r14, 0(%r15)
> +	/* Allocate a stack frame for 16 floating point registers */
> +	/* The size of a FP register is the size of an double word */
> +	slgfi   %r15, 16 * 8
> +	/* Save fp register on stack: offset to SP is multiple of reg number */
> +	.irp i, 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15
> +	std	\i, \i * 8(%r15)
> +	.endr
> +	/* Save fpc, but keep stack aligned on 64bits */
> +	slgfi   %r15, 8
> +	efpc	%r0
> +	stg	%r0, 0(%r15)
> +	.endm

I wonder whether it would be sufficient to only save the registers here
that are "volatile" according to the ELF ABI? ... that would save quite
some space on the stack, I think... OTOH, the old code was also saving
all registers, so maybe that's something for a separate patch later...

Acked-by: Thomas Huth <thuth@redhat.com>