Date: Wed, 5 Jun 2024 13:52:25 +0800
From: Guo Ren <guoren@...nel.org>
To: Cyril Bur <cyrilbur@...storrent.com>
Cc: Anton Blanchard <antonb@...storrent.com>, paul.walmsley@...ive.com, palmer@...belt.com, 
	aou@...s.berkeley.edu, linux-riscv@...ts.infradead.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [CAUTION - External Sender] Re: [PATCH] riscv: Improve exception
 and system call latency

On Tue, Jun 4, 2024 at 4:16 PM Cyril Bur <cyrilbur@...storrent.com> wrote:
>
> On Mon, Jun 3, 2024 at 4:39 PM Guo Ren <guoren@...nel.org> wrote:
> >
> > On Mon, Jun 3, 2024 at 12:38 PM Cyril Bur <cyrilbur@...storrent.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@...nel.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@...storrent.com>
> > > Reviewed-by: Jisheng Zhang <jszhang@...nel.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.
> >
>
> I completely agree with keeping entry.S as clean as possible.
>
> I'm trying to understand what you mean.
> Isn't the point of the patch to remove ra = ret_from_exception?
>
> I'm not sure but maybe we can leave entry.S as it is and the check in
> stacktrace.c can become a check for pc == handle_exception?
Yes, we needn't modify stacktrace.c anymore. Because we still keep "ra
= handle_exception".

>
> >
> > >
> > > 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
> > >
> > >
> > > _______________________________________________
> > > linux-riscv mailing list
> > > linux-riscv@...ts.infradead.org
> > > http://lists.infradead.org/mailman/listinfo/linux-riscv
> >
> >
> >
> > --
> > Best Regards
> >  Guo Ren



-- 
Best Regards
 Guo Ren

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