linux-kernel - Re: [RFC PATCH v8 20/21] riscv: Optimize task switch codes of vector

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Date:   Mon, 4 Oct 2021 22:13:05 +0800
From:   Greentime Hu <greentime.hu@...ive.com>
To:     Jisheng Zhang <jszhang3@...l.ustc.edu.cn>
Cc:     linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Albert Ou <aou@...s.berkeley.edu>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Vincent Chen <vincent.chen@...ive.com>
Subject: Re: [RFC PATCH v8 20/21] riscv: Optimize task switch codes of vector

Jisheng Zhang <jszhang3@...l.ustc.edu.cn> 於 2021年9月15日 週三 下午10:36寫道：
>
> On Thu,  9 Sep 2021 01:45:32 +0800
> Greentime Hu <greentime.hu@...ive.com> wrote:
>
> > This patch replacees 2 instructions with 1 instruction to do the same thing
> > . rs1=x0 with rd != x0 is a special form of the instruction that sets vl to
> > MAXVL.
>
> Similarly, the vector.S is newly introduced in this patch set, so could
> this optimization be folded into the __vstate_save and __vstate_restore
> introduction patch? Or it's better to keep this optimizaion in commit log?
>
Yeah, I wanted to keep the optimization commit log before, but it's ok
if you think merge code is easier to read.
I'll merge this patch in the next version.

> >
> > Suggested-by: Andrew Waterman <andrew@...ive.com>
> > Co-developed-by: Vincent Chen <vincent.chen@...ive.com>
> > Signed-off-by: Vincent Chen <vincent.chen@...ive.com>
> > Signed-off-by: Greentime Hu <greentime.hu@...ive.com>
> > ---
> >  arch/riscv/kernel/vector.S | 9 +++------
> >  1 file changed, 3 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/riscv/kernel/vector.S b/arch/riscv/kernel/vector.S
> > index 4f0c5a166e4e..f7223c81b11a 100644
> > --- a/arch/riscv/kernel/vector.S
> > +++ b/arch/riscv/kernel/vector.S
> > @@ -27,8 +27,7 @@
> >  #define x_vl     t2
> >  #define x_vcsr   t3
> >  #define incr     t4
> > -#define m_one    t5
> > -#define status   t6
> > +#define status   t5
> >
> >  ENTRY(__vstate_save)
> >       li      status, SR_VS
> > @@ -38,8 +37,7 @@ ENTRY(__vstate_save)
> >       csrr    x_vtype, CSR_VTYPE
> >       csrr    x_vl, CSR_VL
> >       csrr    x_vcsr, CSR_VCSR
> > -     li      m_one, -1
> > -     vsetvli incr, m_one, e8, m8
> > +     vsetvli incr, x0, e8, m8
> >       vse8.v   v0, (datap)
> >       add     datap, datap, incr
> >       vse8.v   v8, (datap)
> > @@ -61,8 +59,7 @@ ENTRY(__vstate_restore)
> >       li      status, SR_VS
> >       csrs    CSR_STATUS, status
> >
> > -     li      m_one, -1
> > -     vsetvli incr, m_one, e8, m8
> > +     vsetvli incr, x0, e8, m8
> >       vle8.v   v0, (datap)
> >       add     datap, datap, incr
> >       vle8.v   v8, (datap)
>
>