Re: [PATCH v12 06/17] riscv: Reset vector register

[Guo Ren]

On Fri, Nov 4, 2022 at 1:01 PM Vineet Gupta <vineet.gupta@xxxxxxxxx> wrote:
>
> On 9/21/22 14:43, Chris Stillson wrote:
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >
> > Reset vector registers at boot-time and disable vector instructions
> > execution for kernel mode.
>
> Perhaps bike-shedding, but "Reset" has a different connotation in
> kernel, this is clear registers IMO. And "Reset Vector ..." sounds
> totally different at first glance.
Agree, "Clear vector registers" is okay.

>
>
> > -      * 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
>
> Is VS writable in implementations not implementing V hardware.
>
> Priv spec seems to be confusing. It states
>
>     "The FS[1:0] and VS[1:0] WARL fields..."
>
> Above implies it can be written always but will read legal values only.
> But then this follows.
>
>         "If neither the v registers nor S-mode is implemented, then VS
>         is read-only zero. If S-mode is implemented but the v registers
>         are not, VS may optionally be read-only zero"
>
> What does optionally mean for software ?
The read-only zero bit is safe for writing 1, but the result is still
zero. So let's keep it for easier coding.

>
> >
> >       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 b865046e4dbb..2c81ca42ec4e 100644
> > --- a/arch/riscv/kernel/head.S
> > +++ b/arch/riscv/kernel/head.S
> > @@ -140,10 +140,10 @@ secondary_start_sbi:
> >       .option pop
> >
> >       /*
> > -      * Disable FPU to detect illegal usage of
> > -      * floating point in kernel space
> > +      * Disable FPU & VECTOR to detect illegal usage of
> > +      * floating point or vector in kernel space
> >        */
> > -     li t0, SR_FS
> > +     li t0, SR_FS | SR_VS
> >       csrc CSR_STATUS, t0
> >
> >       /* Set trap vector to spin forever to help debug */
> > @@ -234,10 +234,10 @@ pmp_done:
> >   .option pop
> >
> >       /*
> > -      * Disable FPU to detect illegal usage of
> > -      * floating point in kernel space
> > +      * Disable FPU & VECTOR to detect illegal usage of
> > +      * floating point or vector in kernel space
> >        */
> > -     li t0, SR_FS
> > +     li t0, SR_FS | SR_VS
> >       csrc CSR_STATUS, t0
>
> Third instance of duplicated SR_FS | SR_VS. Better to add a helper
> SR_FS_VS or some such macro.
Good point. But we could move it to another patch and define a new
SR_AXS for all.

#define SR_AXS         (SR_FS | SR_VS | SR_XS)

>
> >
> >   #ifdef CONFIG_RISCV_BOOT_SPINWAIT
> > @@ -431,6 +431,29 @@ 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
> > +     and     t0, t0, t1
> > +     beqz    t0, .Lreset_regs_done
> > +
> > +     /*
> > +      * 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, ta, ma
> > +     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 */
>
> Is that actually happening ?
Yes, It's the same as FPU, see head.S _start_kernel:

ENTRY(_start_kernel)
..
        /* Reset all registers except ra, a0, a1 */
        call reset_regs
...

>
>


-- 
Best Regards
 Guo Ren