Re: [PATCH v9 07/17] riscv: Reset vector register

From: Greentime Hu <greentime.hu@sifive.com>
To: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>,
	linux-riscv <linux-riscv@lists.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Albert Ou <aou@eecs.berkeley.edu>
Subject: Re: [PATCH v9 07/17] riscv: Reset vector register
Date: Tue, 4 Jan 2022 14:02:39 +0800	[thread overview]
Message-ID: <CAHCEehK-oQqsi0fivFfKMUcHrzVoLEGHd29=UKFuGFCBAqCJXw@mail.gmail.com> (raw)
In-Reply-To: <mhng-591e217f-9290-464e-ab17-91fd84bed22b@palmer-ri-x1c9>

Palmer Dabbelt <palmer@dabbelt.com> 於 2021年12月15日 週三 上午12:29寫道：
>
> On Tue, 09 Nov 2021 01:48:19 PST (-0800), greentime.hu@sifive.com wrote:
> > From: Guo Ren <guoren@linux.alibaba.com>
> >
> > Reset vector registers at boot-time and disable vector instructions
> > execution for kernel mode.
> >
> > Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> > Co-developed-by: Vincent Chen <vincent.chen@sifive.com>
> > Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
> > Co-developed-by: Han-Kuan Chen <hankuan.chen@sifive.com>
> > Signed-off-by: Han-Kuan Chen <hankuan.chen@sifive.com>
> > Co-developed-by: Greentime Hu <greentime.hu@sifive.com>
> > Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
> > ---
> >  arch/riscv/kernel/entry.S |  6 +++---
> >  arch/riscv/kernel/head.S  | 22 ++++++++++++++++++++--
> >  2 files changed, 23 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> > index 98f502654edd..ad0fa80ada81 100644
> > --- a/arch/riscv/kernel/entry.S
> > +++ b/arch/riscv/kernel/entry.S
> > @@ -77,10 +77,10 @@ _save_context:
> >        * Disable user-mode memory access as it should only be set in the
> >        * actual user copy routines.
> >        *
> > -      * Disable the FPU to detect illegal usage of floating point in kernel
> > -      * space.
> > +      * Disable the FPU/Vector to detect illegal usage of floating point
> > +      * or vector in kernel space.
> >        */
> > -     li t0, SR_SUM | SR_FS
> > +     li t0, SR_SUM | SR_FS | SR_VS
> >
> >       REG_L s0, TASK_TI_USER_SP(tp)
> >       csrrc s1, CSR_STATUS, t0
> > diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> > index 52c5ff9804c5..551afe1de85e 100644
> > --- a/arch/riscv/kernel/head.S
> > +++ b/arch/riscv/kernel/head.S
> > @@ -242,10 +242,10 @@ pmp_done:
> >  .option pop
> >
> >       /*
> > -      * Disable FPU to detect illegal usage of
> > +      * Disable FPU & VECTOR to detect illegal usage of
> >        * floating point in kernel space
>
> Presumably that should be "floating point or vector", like the other
> one?

Thank you, Palmer. I'll update this and I also found another issue in
secondary_start_sbi.
We should also disable vector there.

> >        */
> > -     li t0, SR_FS
> > +     li t0, SR_FS | SR_VS
> >       csrc CSR_STATUS, t0
> >
> >  #ifdef CONFIG_SMP
> > @@ -433,6 +433,24 @@ ENTRY(reset_regs)
> >       csrw    fcsr, 0
> >       /* note that the caller must clear SR_FS */
> >  #endif /* CONFIG_FPU */
> > +
> > +#ifdef CONFIG_VECTOR
> > +     csrr    t0, CSR_MISA
> > +     li      t1, (COMPAT_HWCAP_ISA_V >> 16)
> > +     slli    t1, t1, 16
>
> Why?  Shouldn't the "li" pseudo handle generating that constant fine?
> It generates the expected lui for me.

That's right.
I'll update the code here to
 #ifdef CONFIG_VECTOR
        csrr    t0, CSR_MISA
        li      t1, COMPAT_HWCAP_ISA_V
        and     t0, t0, t1
        beqz    t0, .Lreset_regs_done

> > +     and     t0, t0, t1
> > +     beqz    t0, .Lreset_regs_done
> > +
> > +     li      t1, SR_VS
> > +     csrs    CSR_STATUS, t1
> > +     vsetvli t1, x0, e8, m8
> > +     vmv.v.i v0, 0
> > +     vmv.v.i v8, 0
> > +     vmv.v.i v16, 0
> > +     vmv.v.i v24, 0
>
> I don't see anything resetting vcsr here, which is explicitly required
> by ISA manual.
>
> Otherwise this looks OK to me: I wasn't actually sure this was guaranteed
> to hit every bit in the vector register file, but IIUC it does -- VLMAX
> has a defined value, VLEN is a constant, and this form of vsetvli is
> defined to set vl to VLMAX.  Probably worth a comment, though.
>

/*
 * Clear vector registers and reset vcsr
 * VLMAX has a defined value, VLEN is a constant,
 * and this form of vsetvli is defined to set vl to VLMAX.
 */
li      t1, SR_VS
csrs    CSR_STATUS, t1
csrs    CSR_VCSR, x0
vsetvli t1, x0, e8, m8
vmv.v.i v0, 0
vmv.v.i v8, 0
vmv.v.i v16, 0
vmv.v.i v24, 0

> > +     /* note that the caller must clear SR_VS */
> > +#endif /* CONFIG_VECTOR */
> > +
> >  .Lreset_regs_done:
> >       ret
> >  END(reset_regs)
>
> With those minor bits fixed,
>
> Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>