[PATCH] riscv: uaccess: Only restore the CSR_STATUS SUM bit

[Andy Chiu]

On Tue, May 27, 2025 at 4:39 AM Alexandre Ghiti <alex at ghiti.fr> wrote:
>
> +cc linux-riscv, Andy, Deepak
>
> On 5/22/25 18:09, Cyril Bur wrote:
> > During switch to csrs will OR the value of the register into the
> > corresponding csr. In this case we're only interested in restoring the
> > SUM bit not the entire register.
> >
> > Fixes: 788aa64c0 ("riscv: save the SR_SUM status over switches")
> > Signed-off-by: Cyril Bur <cyrilbur at tenstorrent.com>
> > ---
> > I've put the Fixes tag in but I assume this will get squashed into the
> > patch. Either way I hope this works to fix the immediate issue.
> >
> >   arch/riscv/kernel/entry.S | 12 ++++++++----
> >   1 file changed, 8 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> > index 00bd0de9faa2..6ed3bd80903d 100644
> > --- a/arch/riscv/kernel/entry.S
> > +++ b/arch/riscv/kernel/entry.S
> > @@ -399,14 +399,18 @@ SYM_FUNC_START(__switch_to)
> >       REG_S s11, TASK_THREAD_S11_RA(a3)
> >
> >       /* save the user space access flag */
> > -     li    s0, SR_SUM
> > -     csrr  s1, CSR_STATUS
> > -     REG_S s1, TASK_THREAD_STATUS_RA(a3)
> > +     csrr  s0, CSR_STATUS
> > +     REG_S s0, TASK_THREAD_STATUS_RA(a3)
> >
> >       /* Save the kernel shadow call stack pointer */
> >       scs_save_current
> > -     /* Restore context from next->thread */
> > +     /*
> > +      * Restore context from next->thread. csrs will OR the bits from s0 and
> > +      * only want to restore the SR_SUM bit
> > +      */
> >       REG_L s0,  TASK_THREAD_STATUS_RA(a4)
> > +     li    s1,  SR_SUM
> > +     and   s0,  s0, s1
> >       csrs  CSR_STATUS, s0
> >       REG_L ra,  TASK_THREAD_RA_RA(a4)
> >       REG_L sp,  TASK_THREAD_SP_RA(a4)
>
> To conclude the discussion we had here
> https://lore.kernel.org/linux-riscv/aDCtATl2N21fBsyT@debug.ba.rivosinc.com/#t,
> in addition to Cyril's patch above, to me we only have to rename the
> status field into sum and we're good to go. @Andy, @Deepak @Samuel Do
> you agree?

LGTM, thanks!

Andy

>
> As this is an important fix (along with 2 other fixes, one for thead
> vector and vdso static values), I'd like to send another PR soon for
> inclusion in 6.16-rc1, I did not want to delay the second PR any longer.
>
> Thanks for your feedbacks,
>
> Alex
>
>