Re: [PATCH v2 5/8] powerpc: add exit_flags field in pt_regs

["Christophe Leroy (CS GROUP)" <chleroy@kernel.org>]

Le 14/12/2025 à 14:02, Mukesh Kumar Chaurasiya a écrit :
> From: Mukesh Kumar Chaurasiya <mchauras@linux.ibm.com>
> 
> Add a new field `exit_flags` in the pt_regs structure. This field will hold
> the flags set during interrupt or syscall execution that are required during
> exit to user mode.
> 
> Specifically, the `TIF_RESTOREALL` flag, stored in this field, helps the
> exit routine determine if any NVGPRs were modified and need to be restored
> before returning to userspace.

In the current implementation we did our best to keep this information 
in a local var for performance reasons. Have you assessed the 
performance impact of going through the stack for that ?

> 
> This addition ensures a clean and architecture-specific mechanism to track
> per-syscall or per-interrupt state transitions related to register restore.
> 
> Changes:
>   - Add `exit_flags` and `__pt_regs_pad` to maintain 16-byte stack alignment
>   - Update asm-offsets.c and ptrace.c for offset and validation
>   - Update PT_* constants in uapi header to reflect the new layout
> 
> Signed-off-by: Mukesh Kumar Chaurasiya <mchauras@linux.ibm.com>
> ---
>   arch/powerpc/include/asm/ptrace.h      |  3 +++
>   arch/powerpc/include/uapi/asm/ptrace.h | 14 +++++++++-----
>   arch/powerpc/kernel/asm-offsets.c      |  1 +
>   arch/powerpc/kernel/ptrace/ptrace.c    |  1 +
>   4 files changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/ptrace.h b/arch/powerpc/include/asm/ptrace.h
> index 94aa1de2b06e..3af8a5898fe3 100644
> --- a/arch/powerpc/include/asm/ptrace.h
> +++ b/arch/powerpc/include/asm/ptrace.h
> @@ -53,6 +53,9 @@ struct pt_regs
>   				unsigned long esr;
>   			};
>   			unsigned long result;
> +			unsigned long exit_flags;
> +			/* Maintain 16 byte interrupt stack alignment */

On powerpc/32, one 'long' is 4 bytes not 8.

> +			unsigned long __pt_regs_pad[1];
>   		};
>   	};
>   #if defined(CONFIG_PPC64) || defined(CONFIG_PPC_KUAP)
> diff --git a/arch/powerpc/include/uapi/asm/ptrace.h b/arch/powerpc/include/uapi/asm/ptrace.h
> index 01e630149d48..de56b216c9c5 100644
> --- a/arch/powerpc/include/uapi/asm/ptrace.h
> +++ b/arch/powerpc/include/uapi/asm/ptrace.h
> @@ -55,6 +55,8 @@ struct pt_regs
>   	unsigned long dar;		/* Fault registers */
>   	unsigned long dsisr;		/* on 4xx/Book-E used for ESR */
>   	unsigned long result;		/* Result of a system call */
> +	unsigned long exit_flags;	/* System call exit flags */
> +	unsigned long __pt_regs_pad[1];	/* Maintain 16 byte interrupt stack alignment */

On powerpc/32, one 'long' is 4 bytes not 8.

>   };
>   
>   #endif /* __ASSEMBLER__ */
> @@ -114,10 +116,12 @@ struct pt_regs
>   #define PT_DAR	41
>   #define PT_DSISR 42
>   #define PT_RESULT 43
> -#define PT_DSCR 44
> -#define PT_REGS_COUNT 44
> +#define PT_EXIT_FLAGS 44
> +#define PT_PAD 45
> +#define PT_DSCR 46
> +#define PT_REGS_COUNT 46
>   
> -#define PT_FPR0	48	/* each FP reg occupies 2 slots in this space */
> +#define PT_FPR0	(PT_REGS_COUNT + 4)	/* each FP reg occupies 2 slots in this space */
>   
>   #ifndef __powerpc64__
>   
> @@ -129,7 +133,7 @@ struct pt_regs
>   #define PT_FPSCR (PT_FPR0 + 32)	/* each FP reg occupies 1 slot in 64-bit space */
>   
>   
> -#define PT_VR0 82	/* each Vector reg occupies 2 slots in 64-bit */
> +#define PT_VR0	(PT_FPSCR + 2)	/* <82> each Vector reg occupies 2 slots in 64-bit */
>   #define PT_VSCR (PT_VR0 + 32*2 + 1)
>   #define PT_VRSAVE (PT_VR0 + 33*2)
>   
> @@ -137,7 +141,7 @@ struct pt_regs
>   /*
>    * Only store first 32 VSRs here. The second 32 VSRs in VR0-31
>    */
> -#define PT_VSR0 150	/* each VSR reg occupies 2 slots in 64-bit */
> +#define PT_VSR0	(PT_VRSAVE + 2)	/* each VSR reg occupies 2 slots in 64-bit */
>   #define PT_VSR31 (PT_VSR0 + 2*31)
>   #endif /* __powerpc64__ */
>   
> diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
> index a4bc80b30410..c0bb09f1db78 100644
> --- a/arch/powerpc/kernel/asm-offsets.c
> +++ b/arch/powerpc/kernel/asm-offsets.c
> @@ -292,6 +292,7 @@ int main(void)
>   	STACK_PT_REGS_OFFSET(_ESR, esr);
>   	STACK_PT_REGS_OFFSET(ORIG_GPR3, orig_gpr3);
>   	STACK_PT_REGS_OFFSET(RESULT, result);
> +	STACK_PT_REGS_OFFSET(EXIT_FLAGS, exit_flags);

Where is that used ?

>   	STACK_PT_REGS_OFFSET(_TRAP, trap);
>   #ifdef CONFIG_PPC64
>   	STACK_PT_REGS_OFFSET(SOFTE, softe);
> diff --git a/arch/powerpc/kernel/ptrace/ptrace.c b/arch/powerpc/kernel/ptrace/ptrace.c
> index c6997df63287..2134b6d155ff 100644
> --- a/arch/powerpc/kernel/ptrace/ptrace.c
> +++ b/arch/powerpc/kernel/ptrace/ptrace.c
> @@ -432,6 +432,7 @@ void __init pt_regs_check(void)
>   	CHECK_REG(PT_DAR, dar);
>   	CHECK_REG(PT_DSISR, dsisr);
>   	CHECK_REG(PT_RESULT, result);
> +	CHECK_REG(PT_EXIT_FLAGS, exit_flags);
>   	#undef CHECK_REG
>   
>   	BUILD_BUG_ON(PT_REGS_COUNT != sizeof(struct user_pt_regs) / sizeof(unsigned long));