Re: [PATCH] riscv: Setup exception vector for K210 properly

[Atish Patra]

On Tue, Aug 11, 2020 at 1:41 AM Anup Patel <anup@xxxxxxxxxxxxxx> wrote:
>
> On Tue, Aug 11, 2020 at 12:07 PM Qiu Wenbo <qiuwenbo@xxxxxxxxxxxxxx> wrote:
> >
> > Exception vector is missing on nommu platform and it is a big issue.
> > This patch is tested in Sipeed MAIX Bit Dev Board.
> >
> > Fixes: 79b1feba5455 ("RISC-V: Setup exception vector early")
> > Signed-off-by: Qiu Wenbo <qiuwenbo@xxxxxxxxxxxxxx>

Thanks for testing it on the kendryte board.

> > ---
> >  arch/riscv/kernel/smpboot.c |  1 +
> >  arch/riscv/kernel/traps.c   | 11 ++++++++++-
> >  2 files changed, 11 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/riscv/kernel/smpboot.c b/arch/riscv/kernel/smpboot.c
> > index 356825a57551..23cde0ceb39d 100644
> > --- a/arch/riscv/kernel/smpboot.c
> > +++ b/arch/riscv/kernel/smpboot.c
> > @@ -154,6 +154,7 @@ asmlinkage __visible void smp_callin(void)
> >         mmgrab(mm);
> >         current->active_mm = mm;
> >
> > +       trap_init();
> >         notify_cpu_starting(curr_cpuid);
> >         update_siblings_masks(curr_cpuid);
> >         set_cpu_online(curr_cpuid, 1);
> > diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> > index ad14f4466d92..a390239818ae 100644
> > --- a/arch/riscv/kernel/traps.c
> > +++ b/arch/riscv/kernel/traps.c
> > @@ -174,7 +174,16 @@ int is_valid_bugaddr(unsigned long pc)
> >  }
> >  #endif /* CONFIG_GENERIC_BUG */
> >
> > -/* stvec & scratch is already set from head.S */
> > +/* stvec & scratch is already set from head.S when mmu is enabled */
> >  void trap_init(void)
> >  {
> > +#ifndef CONFIG_MMU
> > +       /*
> > +        * Set sup0 scratch register to 0, indicating to exception vector
> > +        * that we are presently executing in the kernel
> > +        */
> > +       csr_write(CSR_SCRATCH, 0);
> > +       /* Set the exception vector address */
> > +       csr_write(CSR_TVEC, &handle_exception);
> > +#endif
> >  }
> > --
> > 2.28.0
> >
>
> This issue seems to be only on the latest master branch of
> Linux stable tree so this fix need not be a stable fix.
>
> For MMU kernel, the CSR_TVEC is setup in relocate() function
> called from secondary_start_common() function of head.S
>
> For NoMMU kernel, we should set CSR_TVEC directly in
> secondary_start_common() function as "#else" case of the
> "#ifdef CONFIG_MMU".
>

That would enable the trap only for secondary harts. But the exception
vector on boot hart
is still uninitialized. How about this change ?

diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index d0c5c316e9bb..7822054dbd88 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -77,16 +77,6 @@ relocate:
        csrw CSR_SATP, a0
 .align 2
 1:
-       /* Set trap vector to exception handler */
-       la a0, handle_exception
-       csrw CSR_TVEC, a0
-
-       /*
-        * Set sup0 scratch register to 0, indicating to exception vector that
-        * we are presently executing in kernel.
-        */
-       csrw CSR_SCRATCH, zero
-
        /* Reload the global pointer */
 .option push
 .option norelax
@@ -144,9 +134,23 @@ secondary_start_common:
        la a0, swapper_pg_dir
        call relocate
 #endif
+       call setup_trap_vector
        tail smp_callin
 #endif /* CONFIG_SMP */

+.align 2
+setup_trap_vector:
+       /* Set trap vector to exception handler */
+       la a0, handle_exception
+       csrw CSR_TVEC, a0
+
+       /*
+        * Set sup0 scratch register to 0, indicating to exception vector that
+        * we are presently executing in kernel.
+        */
+       csrw CSR_SCRATCH, zero
+       ret
+
 .Lsecondary_park:
        /* We lack SMP support or have too many harts, so park this hart */
        wfi
@@ -240,6 +244,7 @@ clear_bss_done:
        call relocate
 #endif /* CONFIG_MMU */

+       call setup_trap_vector
        /* Restore C environment */
        la tp, init_task
        sw zero, TASK_TI_CPU(tp)


> Regards,
> Anup
>
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-- 
Regards,
Atish