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SubjectRe: [PATCH] [RFC] aarch64: scs: reload shadow call stack in user exception entry
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On 1/4/22 2:40 AM, Mark Rutland wrote:
> Hi,
>
> On Mon, Dec 27, 2021 at 05:06:04PM -0800, Dan Li wrote:
>> When el0 exception occurs, kernel_entry/exit will load/save tsk->scs_sp
>> to ensure scs working properly. AFAIK, the SCS offset should always be
>> 0 at this time.
>>
>> Is it reasonable to reload x18 to scs_base directly in kernel_entry
>> here, or am I missing something?
>>
>> Signed-off-by: Dan Li <ashimida@linux.alibaba.com>
>
> AFAICT the saving here is just that we avoid storing the SCS SP upon return to
> EL0, and I suspect that in practice that's not measureable.
>
> So without numbers to show otherwise, I'd prefer to leave this as-is, though I
> agree that this patch should work. If we do want to change this, I think we
> need a comment in the `kernel_exit` path to pair with the `scs_load` in
> `kernel_entry`.
>
> Thanks,
> Mark.
>
Yes, there is only one instruction saved here, which should have a
minimal impact on performance.
The current code is also good for me :)

Thanks,
Dan.

>> ---
>> arch/arm64/include/asm/scs.h | 7 +++++++
>> arch/arm64/kernel/entry.S | 3 +--
>> 2 files changed, 8 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/scs.h b/arch/arm64/include/asm/scs.h
>> index 8297bccf0784..2bc0d0575e75 100644
>> --- a/arch/arm64/include/asm/scs.h
>> +++ b/arch/arm64/include/asm/scs.h
>> @@ -9,6 +9,10 @@
>> #ifdef CONFIG_SHADOW_CALL_STACK
>> scs_sp .req x18
>>
>> + .macro scs_reload tsk
>> + ldr scs_sp, [\tsk, #TSK_TI_SCS_BASE]
>> + .endm
>> +
>> .macro scs_load tsk
>> ldr scs_sp, [\tsk, #TSK_TI_SCS_SP]
>> .endm
>> @@ -17,6 +21,9 @@
>> str scs_sp, [\tsk, #TSK_TI_SCS_SP]
>> .endm
>> #else
>> + .macro scs_reload tsk
>> + .endm
>> +
>> .macro scs_load tsk
>> .endm
>>
>> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
>> index bc6d5a970a13..57547a3e4f7c 100644
>> --- a/arch/arm64/kernel/entry.S
>> +++ b/arch/arm64/kernel/entry.S
>> @@ -265,7 +265,7 @@ alternative_if ARM64_HAS_ADDRESS_AUTH
>> alternative_else_nop_endif
>> 1:
>>
>> - scs_load tsk
>> + scs_reload tsk
>> .else
>> add x21, sp, #PT_REGS_SIZE
>> get_current_task tsk
>> @@ -365,7 +365,6 @@ alternative_if ARM64_WORKAROUND_845719
>> alternative_else_nop_endif
>> #endif
>> 3:
>> - scs_save tsk
>>
>> /* Ignore asynchronous tag check faults in the uaccess routines */
>> ldr x0, [tsk, THREAD_SCTLR_USER]
>> --
>> 2.17.1
>>

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