Messages in this thread | | | Subject | Re: [PATCH] [RFC] aarch64: scs: reload shadow call stack in user exception entry | From | Dan Li <> | Date | Tue, 4 Jan 2022 06:32:16 -0800 |
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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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