[PATCH] riscv: Improve exception and system call latency

[Guo Ren]

On Mon, Jun 3, 2024 at 12:38 PM Cyril Bur <cyrilbur at tenstorrent.com> wrote:
>
> [ apologies, I think my mailer is going to mess up the formatting ]
>
> On 26 Dec 2023, at 2:56 PM, Guo Ren <guoren at kernel.org> wrote:
>
> On Sun, Dec 24, 2023 at 08:00:18PM -0800, Anton Blanchard wrote:
>
> Many CPUs implement return address branch prediction as a stack. The
> RISCV architecture refers to this as a return address stack (RAS). If
> this gets corrupted then the CPU will mispredict at least one but
> potentally many function returns.
>
> There are two issues with the current RISCV exception code:
>
> - We are using the alternate link stack (x5/t0) for the indirect branch
>  which makes the hardware think this is a function return. This will
>  corrupt the RAS.
>
> - We modify the return address of handle_exception to point to
>  ret_from_exception. This will also corrupt the RAS.
>
> Testing the null system call latency before and after the patch:
>
> Visionfive2 (StarFive JH7110 / U74)
> baseline: 189.87 ns
> patched:  176.76 ns
>
> Lichee pi 4a (T-Head TH1520 / C910)
> baseline: 666.58 ns
> patched:  636.90 ns
>
> Just over 7% on the U74 and just over 4% on the C910.
>
>
> Yes, the wrong "jalr zero, t0/ra" would pop RAS and destroy the RAS
> layout of the hardware for the userspace. How about giving a fake push
> for the RAS to connect "jalr zero, ra" of sub-function call return? I'm
> curious if you could measure the difference with only one RAS
> misprediction.
>
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index 54ca4564a926..94c7d2be35d0 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -93,7 +93,8 @@ SYM_CODE_START(handle_exception)
>        bge s4, zero, 1f
>
>        /* Handle interrupts */
> -       tail do_irq
> +       auipc t0, do_irq
> +       jalr  t0, t0
> 1:
>        /* Handle other exceptions */
>        slli t0, s4, RISCV_LGPTR
> @@ -103,9 +104,10 @@ SYM_CODE_START(handle_exception)
>        /* Check if exception code lies within bounds */
>        bgeu t0, t2, 1f
>        REG_L t0, 0(t0)
> -       jr t0
> +       jalr t0, t0
> 1:
> -       tail do_trap_unknown
> +       auipc t0, do_trap_unknown
> +       jalr  t0, t0
> SYM_CODE_END(handle_exception)
>
> You could prepare a deeper userspace stack calling if you want better
> measurement results.
>
>
> Signed-off-by: Anton Blanchard <antonb at tenstorrent.com>
> Reviewed-by: Jisheng Zhang <jszhang at kernel.org>
> ---
>
> This introduces some complexity in the stackframe walk code. PowerPC
> resolves the multiple exception exit paths issue by placing a value into
> the exception stack frame (basically the word "REGS") that the stack frame
> code can look for. Perhaps something to look at.
>
> arch/riscv/kernel/entry.S      | 21 ++++++++++++++-------
> arch/riscv/kernel/stacktrace.c | 14 +++++++++++++-
> 2 files changed, 27 insertions(+), 8 deletions(-)
>
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index 54ca4564a926..89af35edbf6c 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -84,7 +84,6 @@ SYM_CODE_START(handle_exception)
> scs_load_current_if_task_changed s5
>
> move a0, sp /* pt_regs */
> - la ra, ret_from_exception
>
> /*
> * MSB of cause differentiates between
> @@ -93,7 +92,10 @@ SYM_CODE_START(handle_exception)
> bge s4, zero, 1f
>
> /* Handle interrupts */
> - tail do_irq
> + call do_irq
> +.globl ret_from_irq_exception
> +ret_from_irq_exception:
> + j ret_from_exception
> 1:
> /* Handle other exceptions */
> slli t0, s4, RISCV_LGPTR
> @@ -101,11 +103,16 @@ SYM_CODE_START(handle_exception)
> la t2, excp_vect_table_end
> add t0, t1, t0
> /* Check if exception code lies within bounds */
> - bgeu t0, t2, 1f
> - REG_L t0, 0(t0)
> - jr t0
> -1:
> - tail do_trap_unknown
> + bgeu t0, t2, 3f
> + REG_L t1, 0(t0)
> +2: jalr ra,t1
> +.globl ret_from_other_exception
> +ret_from_other_exception:
> + j ret_from_exception
> +3:
> +
> + la t1, do_trap_unknown
> + j 2b
> SYM_CODE_END(handle_exception)
>
> /*
> diff --git a/arch/riscv/kernel/stacktrace.c b/arch/riscv/kernel/stacktrace.c
> index 64a9c093aef9..b9cd131bbc4c 100644
> --- a/arch/riscv/kernel/stacktrace.c
> +++ b/arch/riscv/kernel/stacktrace.c
> @@ -17,6 +17,18 @@
> #ifdef CONFIG_FRAME_POINTER
>
> extern asmlinkage void ret_from_exception(void);
> +extern asmlinkage void ret_from_irq_exception(void);
> +extern asmlinkage void ret_from_other_exception(void);
> +
> +static inline bool is_exception_frame(unsigned long pc)
> +{
> + if ((pc == (unsigned long)ret_from_exception) ||
> +    (pc == (unsigned long)ret_from_irq_exception) ||
> +    (pc == (unsigned long)ret_from_other_exception))
> + return true;
> +
> + return false;
> +}
>
> We needn't put too many .globl in the entry.S, and just check that pc is
> in SYM_CODE_START/END(handle_exception), then entry.S would be cleaner:
>
> Hi Guo,
>
> I've taken this patch over from Anton, mostly just to tidy it up. I'd
> like to incorporate
> what you mention here but I'm not sure how to achieve it. Have I
> missed something
> obvious? As things currently stand there doesn't seem to be a way to get the end
> (or size) of handle_exception in C code.
"just check that pc is in SYM_CODE_START/END(handle_exception)."
Sorry, I think my previous description is wrong.

Instead, "We needn't modify anything in stacktrace.c because we keep
ra = ret_from_exception."

I want only cleaner and smaller modifications to the entry.S to
satisfy RAS prediction performance requirements.


>
> Your advice is greatly appreciated,
>
> Thanks,
>
> Cyril
>
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index 54ca4564a926..d452d5f12b1b 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -84,7 +84,6 @@ SYM_CODE_START(handle_exception)
>        scs_load_current_if_task_changed s5
>
>        move a0, sp /* pt_regs */
>
>        /*
>         * MSB of cause differentiates between
> @@ -93,7 +92,8 @@ SYM_CODE_START(handle_exception)
>        bge s4, zero, 1f
>
>        /* Handle interrupts */
>        call do_irq
>        j ret_from_exception
> 1:
>        /* Handle other exceptions */
>        slli t0, s4, RISCV_LGPTR
> @@ -102,10 +102,12 @@ SYM_CODE_START(handle_exception)
>        add t0, t1, t0
>        /* Check if exception code lies within bounds */
>        bgeu t0, t2, 1f
>        REG_L ra, 0(t0)
>        jalr ra, ra
>        j ret_from_exception
> 1:
>        call do_trap_unknown
>        j ret_from_exception
> SYM_CODE_END(handle_exception)
>
>
>
> void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs,
>     bool (*fn)(void *, unsigned long), void *arg)
> @@ -62,7 +74,7 @@ void notrace walk_stackframe(struct task_struct
> *task, struct pt_regs *regs,
> fp = frame->fp;
> pc = ftrace_graph_ret_addr(current, NULL, frame->ra,
>   &frame->ra);
> - if (pc == (unsigned long)ret_from_exception) {
> + if (is_exception_frame(pc)) {
> if (unlikely(!__kernel_text_address(pc) || !fn(arg, pc)))
> break;
>
> --
> 2.25.1
>
>
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-- 
Best Regards
 Guo Ren