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Message-ID: <aDifiKsdE-29w7-N@debug.ba.rivosinc.com>
Date: Thu, 29 May 2025 10:55:20 -0700
From: Deepak Gupta <debug@...osinc.com>
To: Andy Chiu <andybnac@...il.com>
Cc: Alexandre Ghiti <alex@...ti.fr>, Cyril Bur <cyrilbur@...storrent.com>,
samuel.holland@...ive.com, ben.dooks@...ethink.co.uk,
palmer@...belt.com, linux-kernel@...r.kernel.org,
jszhang@...nel.org, paul.walmsley@...ive.com, charlie@...osinc.com,
jrtc27@...c27.com, aou@...s.berkeley.edu,
linux-riscv@...ts.infradead.org
Subject: Re: [PATCH] riscv: uaccess: Only restore the CSR_STATUS SUM bit
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@...ti.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@...storrent.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
>>
>>
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