Re: [PATCH v2 1/1] microblaze: allocate stack space for register params

[Michal Simek <michal.simek@amd.com>]

On 4/21/26 14:21, Neal Frager wrote:
> C functions are invoked with arguments without allocating stack space for
> register params (for potential register spills). As a result kernel built
> with gcc v15 boot hangs at the point of starting init.
> 
> Add necessary stack space for few C function calls from entry.S
> (not all). With this change, the kernel boots all the way to login prompt
> and root login works. Simple commands like ps work and show normal
> output.
> 
> Fixes: ca54502bd52a ("microblaze_mmu_v2: entry.S, entry.h")
> 
> From: Gopi Kumar Bulusu <gopi@sankhya.com>
> Co-developed-by: Neal Frager <neal.frager@amd.com>
> Signed-off-by: Neal Frager <neal.frager@amd.com>
> ---
> V1->V2:
> - removed whitespaces
> - changed ret_from_trap2 to ret_from_trap_noadj
> - added comment for SYSCALL_ARGS_SIZE
> - cleaned up language in commit message
> ---
>   arch/microblaze/kernel/entry.S | 21 +++++++++++++++------
>   1 file changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/microblaze/kernel/entry.S b/arch/microblaze/kernel/entry.S
> index 582d7256d815..30d42b84f156 100644
> --- a/arch/microblaze/kernel/entry.S
> +++ b/arch/microblaze/kernel/entry.S
> @@ -339,6 +339,10 @@ C_ENTRY(mb_invalidate_icache):
>    * Trap entered via brki instruction, so BIP bit is set, and interrupts
>    * are masked. This is nice, means we don't have to CLI before state save
>    */
> +
> +/* Syscall args in r5-r10 and the link register or 28 bytes */
> +#define SYSCALL_ARGS_SIZE 28
> +
>   C_ENTRY(_user_exception):
>   	swi	r1, r0, TOPHYS(PER_CPU(ENTRY_SP)) /* save stack */
>   	addi	r14, r14, 4	/* return address is 4 byte after call */
> @@ -422,16 +426,19 @@ C_ENTRY(_user_exception):
>   	lwi	r12, r12, sys_call_table
>   	/* where the trap should return need -8 to adjust for rtsd r15, 8 */
>   	addi	r15, r0, ret_from_trap-8
> +	addik	r1, r1, -SYSCALL_ARGS_SIZE
>   	bra	r12
>   
>   	/* The syscall number is invalid, return an error.  */
>   5:
> -	braid	ret_from_trap
> +	braid	ret_from_trap_noadj
>   	addi	r3, r0, -ENOSYS;
>   
>   /* Entry point used to return from a syscall/trap */
>   /* We re-enable BIP bit before state restore */
>   C_ENTRY(ret_from_trap):
> +	addik	r1, r1, SYSCALL_ARGS_SIZE
> +C_ENTRY(ret_from_trap_noadj):
>   	swi	r3, r1, PT_R3
>   	swi	r4, r1, PT_R4
>   
> @@ -504,7 +511,7 @@ C_ENTRY(ret_from_fork):
>   	bralid	r15, schedule_tail; /* ...which is schedule_tail's arg */
>   	add	r5, r3, r0;	/* switch_thread returns the prev task */
>   				/* ( in the delay slot ) */
> -	brid	ret_from_trap;	/* Do normal trap return */
> +	brid	ret_from_trap_noadj; /* Do normal trap return */
>   	add	r3, r0, r0;	/* Child's fork call should return 0. */
>   
>   C_ENTRY(ret_from_kernel_thread):
> @@ -513,13 +520,13 @@ C_ENTRY(ret_from_kernel_thread):
>   				/* ( in the delay slot ) */
>   	brald	r15, r20	/* fn was left in r20 */
>   	addk	r5, r0, r19	/* ... and argument - in r19 */
> -	brid	ret_from_trap
> +	brid	ret_from_trap_noadj
>   	add	r3, r0, r0
>   
>   C_ENTRY(sys_rt_sigreturn_wrapper):
>   	addik	r30, r0, 0		/* no restarts */
>   	brid	sys_rt_sigreturn	/* Do real work */
> -	addik	r5, r1, 0;		/* add user context as 1st arg */
> +	addik	r5, r1, SYSCALL_ARGS_SIZE;	/* add user context as 1st arg */
>   
>   /*
>    * HW EXCEPTION rutine start
> @@ -729,11 +736,13 @@ C_ENTRY(_interrupt):
>   	lwi	CURRENT_TASK, r0, TOPHYS(PER_CPU(CURRENT_SAVE));
>   	tovirt(r1,r1)
>   	addik	r15, r0, irq_call;
> +	addik	r1, r1, -SYSCALL_ARGS_SIZE
>   irq_call:rtbd	r0, do_IRQ;
> -	addik	r5, r1, 0;
> +	addik	r5, r1, SYSCALL_ARGS_SIZE;
>   
>   /* MS: we are in virtual mode */
>   ret_from_irq:
> +	addik	r1, r1, SYSCALL_ARGS_SIZE
>   	lwi	r11, r1, PT_MODE;
>   	bnei	r11, 2f;
>   
> @@ -1302,7 +1311,7 @@ type_SYSCALL_PREEMPT:
>   .global microblaze_trap_handlers
>   microblaze_trap_handlers:
>   	/* Exact matches come first */
> -	.word ret_from_trap; .word ret_from_trap   ; .word type_SYSCALL
> +	.word ret_from_trap; .word ret_from_trap_noadj  ; .word type_SYSCALL
>   	.word ret_from_irq ; .word ret_from_irq    ; .word type_IRQ
>   	/* Fuzzy matches go here */
>   	.word ret_from_irq ; .word no_intr_resched ; .word type_IRQ_PREEMPT

For a record this patch has be replaced by
https://lore.kernel.org/all/20260821151809.1233057-1-ramin.moussavi@yacoub.de/

Thanks,
Michal