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Message-ID: <es7wva5ptfty5lzrvugew2q2jyb5xygz6tbdp5ztklnfvrhwzl@gqvzutk7whvr>
Date: Tue, 16 Dec 2025 20:26:30 +0530
From: Mukesh Kumar Chaurasiya <mkchauras@...ux.ibm.com>
To: "Christophe Leroy (CS GROUP)" <chleroy@...nel.org>
Cc: maddy@...ux.ibm.com, mpe@...erman.id.au, npiggin@...il.com,
oleg@...hat.com, kees@...nel.org, luto@...capital.net,
wad@...omium.org, mchauras@...ux.ibm.com, thuth@...hat.com,
sshegde@...ux.ibm.com, charlie@...osinc.com, macro@...am.me.uk,
akpm@...ux-foundation.org, ldv@...ace.io, deller@....de,
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tglx@...utronix.de, thomas.weissschuh@...utronix.de,
peterz@...radead.org, menglong8.dong@...il.com, bigeasy@...utronix.de,
namcao@...utronix.de, kan.liang@...ux.intel.com, mingo@...nel.org,
atrajeev@...ux.vnet.ibm.com, mark.barnett@....com,
linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/8] powerpc: add exit_flags field in pt_regs
On Tue, Dec 16, 2025 at 10:52:42AM +0100, Christophe Leroy (CS GROUP) wrote:
>
>
> Le 14/12/2025 à 14:02, Mukesh Kumar Chaurasiya a écrit :
> > From: Mukesh Kumar Chaurasiya <mchauras@...ux.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 ?
>
I needed this information out of the stack calls so kept it here. After
enabling the code as a whole i didn't see much of an impact.
> >
> > 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@...ux.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.
>
Oh ohk. Will fix this in next revision.
> > + 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.
>
Will fix this too.
> > };
> > #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 ?
>
It's not used anywhere as of now but kept it there as a convention.
Should this be removed?
Regards,
Mukesh
> > 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));
>
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