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

[Deepak Gupta <debug@rivosinc.com>]

On Thu, May 29, 2025 at 12:17:24AM +0800, Andy Chiu wrote:
>On Tue, May 27, 2025 at 4:39 AM Alexandre Ghiti <alex@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@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

Sounds good to me.

>
>>
>> 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
>>
>>