Re: [PATCH -next V12 3/7] riscv: entry: Add noinstr to prevent instrumentation inserted

[Guo Ren]

On Tue, Jan 3, 2023 at 5:12 PM Alexandre Ghiti <alex@xxxxxxxx> wrote:
>
> Hi Guo,
>
> On 1/3/23 04:35, guoren@xxxxxxxxxx wrote:
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >
> > Without noinstr the compiler is free to insert instrumentation (think
> > all the k*SAN, KCov, GCov, ftrace etc..) which can call code we're not
> > yet ready to run this early in the entry path, for instance it could
> > rely on RCU which isn't on yet, or expect lockdep state. (by peterz)
> >
> > Link: https://lore.kernel.org/linux-riscv/YxcQ6NoPf3AH0EXe@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> > Reviewed-by: Björn Töpel <bjorn@xxxxxxxxxxxx>
> > Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Tested-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> > ---
> >   arch/riscv/kernel/traps.c | 4 ++--
> >   arch/riscv/mm/fault.c     | 2 +-
> >   2 files changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> > index 549bde5c970a..96ec76c54ff2 100644
> > --- a/arch/riscv/kernel/traps.c
> > +++ b/arch/riscv/kernel/traps.c
> > @@ -95,9 +95,9 @@ static void do_trap_error(struct pt_regs *regs, int signo, int code,
> >   }
> >
> >   #if defined(CONFIG_XIP_KERNEL) && defined(CONFIG_RISCV_ALTERNATIVE)
> > -#define __trap_section               __section(".xip.traps")
> > +#define __trap_section __noinstr_section(".xip.traps")
> >   #else
> > -#define __trap_section
> > +#define __trap_section noinstr
> >   #endif
> >   #define DO_ERROR_INFO(name, signo, code, str)                               \
> >   asmlinkage __visible __trap_section void name(struct pt_regs *regs) \
> > diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
> > index d86f7cebd4a7..b26f68eac61c 100644
> > --- a/arch/riscv/mm/fault.c
> > +++ b/arch/riscv/mm/fault.c
> > @@ -204,7 +204,7 @@ static inline bool access_error(unsigned long cause, struct vm_area_struct *vma)
> >    * This routine handles page faults.  It determines the address and the
> >    * problem, and then passes it off to one of the appropriate routines.
> >    */
> > -asmlinkage void do_page_fault(struct pt_regs *regs)
> > +asmlinkage void noinstr do_page_fault(struct pt_regs *regs)
>
>
> (I dug the archive but can't find the series before v4, so sorry if it
> was already answered)
>
> I think we should not disable the instrumentation of those trap handlers
> as at least profiling them with ftrace would provide valuable
> information (and gcov would be nice too): why do we need to do that? A
> trap very early in the boot process is not recoverable anyway.
Everything that calls irqentry_enter() should be noinstr, and this
patch prepares for the next generic_entry convert.

eg:
asmlinkage void noinstr do_page_fault(struct pt_regs *regs)
{
        irqentry_state_t state = irqentry_enter(regs);

        __do_page_fault(regs);

        local_irq_disable();

        irqentry_exit(regs, state);
}
NOKPROBE_SYMBOL(do_page_fault);

You still could profile __do_page_fault.

>
> And I took a look at other architectures, none of them disables the
> instrumentation on do_page_fault.
That's not true, have a look at power & arm64. All of them have some
limitations at the entry of page_fault.

>
>
> >   {
> >       struct task_struct *tsk;
> >       struct vm_area_struct *vma;



-- 
Best Regards
 Guo Ren