Re: [PATCH] riscv: xip: support runtime trap patching

[Vitaly Wool]

Hi Alex,

On Fri, Jun 4, 2021 at 11:35 AM Alex Ghiti <alex@xxxxxxxx> wrote:
>
> Le 31/05/2021 à 17:17, Alex Ghiti a écrit :
> > Hi Vitaly,
> >
> > Le 31/05/2021 à 10:53, Vitaly Wool a écrit :
> >> RISCV_ERRATA_ALTERNATIVE patches text at runtime which is currently
> >> not possible when the kernel is executed from the flash in XIP mode.
> >> Since runtime patching concerns only traps at the moment, let's just
> >> have all the traps reside in RAM anyway if RISCV_ERRATA_ALTERNATIVE
> >> is set. Thus, these functions will be patch-able even when the .text
> >> section is in flash.
> >>
> >
> > This sounds like a good fix for sifive platforms to work with XIP kernel
> > in 5.13: did you test that it actually works on HW?
> >
> >> Signed-off-by: Vitaly Wool <vitaly.wool@xxxxxxxxxxxx>
> >> ---
> >>   arch/riscv/kernel/traps.c           | 13 +++++++++----
> >>   arch/riscv/kernel/vmlinux-xip.lds.S | 15 ++++++++++++++-
> >>   2 files changed, 23 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> >> index 0721b9798595..7bc88d8aab97 100644
> >> --- a/arch/riscv/kernel/traps.c
> >> +++ b/arch/riscv/kernel/traps.c
> >> @@ -86,8 +86,13 @@ static void do_trap_error(struct pt_regs *regs, int
> >> signo, int code,
> >>       }
> >>   }
> >> +#if defined (CONFIG_XIP_KERNEL) && defined
> >> (CONFIG_RISCV_ERRATA_ALTERNATIVE)
> >> +#define __trap_section        __section(".xip.traps")
> >> +#else
> >> +#define __trap_section
> >> +#endif
> >
> > Maybe we could do something more generic. At the moment, only traps are
> > subject to alternatives but that will likely expand: what about rather
> > defining a section called __alternative_section?
>
> Any thoughts about that?

Oh sorry, I have overlooked this. I do agree this is the way to go
long term, I just wanted to do a reasonably good quick fix since the
default SiFive configuration won't even build with CONFIG_XIP enabled.
So I would probably just suggest to take the existing solution and
then think over the details (e. g. should we just use one "alternative
section" for both the functions and the table or have separate ones?)
for the generic solution.

Best regards,
   Vitaly

> Thanks,
>
> Alex
>
> >
> >>   #define DO_ERROR_INFO(name, signo, code, str)                \
> >> -asmlinkage __visible void name(struct pt_regs *regs)            \
> >> +asmlinkage __visible __trap_section void name(struct pt_regs *regs)    \
> >>   {                                    \
> >>       do_trap_error(regs, signo, code, regs->epc, "Oops - " str);    \
> >>   }
> >> @@ -111,7 +116,7 @@ DO_ERROR_INFO(do_trap_store_misaligned,
> >>   int handle_misaligned_load(struct pt_regs *regs);
> >>   int handle_misaligned_store(struct pt_regs *regs);
> >> -asmlinkage void do_trap_load_misaligned(struct pt_regs *regs)
> >> +asmlinkage void __trap_section do_trap_load_misaligned(struct pt_regs
> >> *regs)
> >>   {
> >>       if (!handle_misaligned_load(regs))
> >>           return;
> >> @@ -119,7 +124,7 @@ asmlinkage void do_trap_load_misaligned(struct
> >> pt_regs *regs)
> >>                 "Oops - load address misaligned");
> >>   }
> >> -asmlinkage void do_trap_store_misaligned(struct pt_regs *regs)
> >> +asmlinkage void __trap_section do_trap_store_misaligned(struct
> >> pt_regs *regs)
> >>   {
> >>       if (!handle_misaligned_store(regs))
> >>           return;
> >> @@ -146,7 +151,7 @@ static inline unsigned long
> >> get_break_insn_length(unsigned long pc)
> >>       return GET_INSN_LENGTH(insn);
> >>   }
> >> -asmlinkage __visible void do_trap_break(struct pt_regs *regs)
> >> +asmlinkage __visible __trap_section void do_trap_break(struct pt_regs
> >> *regs)
> >>   {
> >>   #ifdef CONFIG_KPROBES
> >>       if (kprobe_single_step_handler(regs))
> >> diff --git a/arch/riscv/kernel/vmlinux-xip.lds.S
> >> b/arch/riscv/kernel/vmlinux-xip.lds.S
> >> index 4b29b9917f99..a3ff09c4c3f9 100644
> >> --- a/arch/riscv/kernel/vmlinux-xip.lds.S
> >> +++ b/arch/riscv/kernel/vmlinux-xip.lds.S
> >> @@ -99,9 +99,22 @@ SECTIONS
> >>       }
> >>       PERCPU_SECTION(L1_CACHE_BYTES)
> >> -    . = ALIGN(PAGE_SIZE);
> >> +    . = ALIGN(8);
> >> +    .alternative : {
> >> +        __alt_start = .;
> >> +        *(.alternative)
> >> +        __alt_end = .;
> >> +    }
> >>       __init_end = .;
> >> +    . = ALIGN(16);
> >
> > Why 16 here?
> >
> >> +    .xip.traps : {
> >> +        __xip_traps_start = .;
> >> +        *(.xip.traps)
> >> +        __xip_traps_end = .;
> >> +    }
> >> +
> >> +    . = ALIGN(PAGE_SIZE);
> >>       .sdata : {
> >>           __global_pointer$ = . + 0x800;
> >>           *(.sdata*)
> >>
> >
> > Thanks,
> >
> > Alex
> >
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