Re: [PATCH v8 20/29] s390: Introduce Start Arm Execution instruction

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] s390: missing `nop` padding after SAE instruction execution
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commit 97e095ed7008049c3b17921733022a1015bd2720
Author: Steffen Eiden <seiden@linux.ibm.com>

s390: Introduce Start Arm Execution instruction

This commit introduces the Start Arm Execution (SAE) instruction to enable
accelerated execution of arm64 virtual machines on s390. It adds the
necessary assembly wrapper __sae64a, control blocks, and architecture
types.

> diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
> --- a/arch/s390/kernel/entry.S
> +++ b/arch/s390/kernel/entry.S

[ ... ]

> @@ -245,6 +245,28 @@ EXPORT_SYMBOL(__sie64a)
>  EXPORT_SYMBOL(sie_exit)
>  #endif
>  
> +#if IS_ENABLED(CONFIG_KVM)
> +/*
> + * __sae64a calling convention:
> + * %r2 pointer to sae control block physical address
> + */
> +SYM_FUNC_START(__sae64a)
> +	stmg	%r6,%r14,__SF_GPRS(%r15)	# store kernel registers
> +	STBEAR	__SF_SAE_BEAR(%r15)		# save breaking event address register
> +	.insn	rre,0xb9a50000,%r2,0		# Start Arm Execution
> +	LBEAR	__SF_SAE_BEAR(%r15)		# restore breaking event address register

[Severity: Medium]
Should there be a nopr 7 padding inserted after the SAE instruction here?

For the SIE instruction, the handler explicitly includes a nopr 7 cushion 
because if a machine check is triggered by the immediately following instruction
(in this case LBEAR), the hardware may incorrectly attribute the exception to
the guest context instead of the host.

Since the hardware behavior for SAE likely mirrors SIE, does this path require
similar padding to safely handle machine-check boundaries?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260918133107.1042730-1-seiden@linux.ibm.com?part=20