Re: [PATCH 16/17] riscv: clear the instruction cache and all registers when booting

[Atish Patra]

On Mon, 2019-06-24 at 07:43 +0200, Christoph Hellwig wrote:
> When we get booted we want a clear slate without any leaks from
> previous
> supervisors or the firmware.  Flush the instruction cache and then
> clear
> all registers to known good values.  This is really important for the
> upcoming nommu support that runs on M-mode, but can't really harm
> when
> running in S-mode either.

That means it should be done for S-mode as well. Right ?
I see the reset code is enabled only for M-mode only.

>   Vaguely based on the concepts from opensbi.
> 
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
>  arch/riscv/kernel/head.S | 85
> ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 85 insertions(+)
> 
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index a4c170e41a34..74feb17737b4 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -11,6 +11,7 @@
>  #include <asm/thread_info.h>
>  #include <asm/page.h>
>  #include <asm/csr.h>
> +#include <asm/hwcap.h>
>  
>  __INIT
>  ENTRY(_start)
> @@ -19,6 +20,12 @@ ENTRY(_start)
>  	csrw CSR_XIP, zero
>  
>  #ifdef CONFIG_M_MODE
> +	/* flush the instruction cache */
> +	fence.i
> +
> +	/* Reset all registers except ra, a0, a1 */
> +	call reset_regs
> +
>  	/*
>  	 * The hartid in a0 is expected later on, and we have no
> firmware
>  	 * to hand it to us.
> @@ -168,6 +175,84 @@ relocate:
>  	j .Lsecondary_park
>  END(_start)
>  
> +#ifdef CONFIG_M_MODE
> +ENTRY(reset_regs)
> +	li	sp, 0
> +	li	gp, 0
> +	li	tp, 0
> +	li	t0, 0
> +	li	t1, 0
> +	li	t2, 0
> +	li	s0, 0
> +	li	s1, 0
> +	li	a2, 0
> +	li	a3, 0
> +	li	a4, 0
> +	li	a5, 0
> +	li	a6, 0
> +	li	a7, 0
> +	li	s2, 0
> +	li	s3, 0
> +	li	s4, 0
> +	li	s5, 0
> +	li	s6, 0
> +	li	s7, 0
> +	li	s8, 0
> +	li	s9, 0
> +	li	s10, 0
> +	li	s11, 0
> +	li	t3, 0
> +	li	t4, 0
> +	li	t5, 0
> +	li	t6, 0
> +	csrw	sscratch, 0
> +
> +#ifdef CONFIG_FPU
> +	csrr	t0, misa
> +	andi	t0, t0, (COMPAT_HWCAP_ISA_F | COMPAT_HWCAP_ISA_D)
> +	bnez	t0, .Lreset_regs_done
> +
> +	li	t1, SR_FS
> +	csrs	sstatus, t1
> +	fmv.s.x	f0, zero
> +	fmv.s.x	f1, zero
> +	fmv.s.x	f2, zero
> +	fmv.s.x	f3, zero
> +	fmv.s.x	f4, zero
> +	fmv.s.x	f5, zero
> +	fmv.s.x	f6, zero
> +	fmv.s.x	f7, zero
> +	fmv.s.x	f8, zero
> +	fmv.s.x	f9, zero
> +	fmv.s.x	f10, zero
> +	fmv.s.x	f11, zero
> +	fmv.s.x	f12, zero
> +	fmv.s.x	f13, zero
> +	fmv.s.x	f14, zero
> +	fmv.s.x	f15, zero
> +	fmv.s.x	f16, zero
> +	fmv.s.x	f17, zero
> +	fmv.s.x	f18, zero
> +	fmv.s.x	f19, zero
> +	fmv.s.x	f20, zero
> +	fmv.s.x	f21, zero
> +	fmv.s.x	f22, zero
> +	fmv.s.x	f23, zero
> +	fmv.s.x	f24, zero
> +	fmv.s.x	f25, zero
> +	fmv.s.x	f26, zero
> +	fmv.s.x	f27, zero
> +	fmv.s.x	f28, zero
> +	fmv.s.x	f29, zero
> +	fmv.s.x	f30, zero
> +	fmv.s.x	f31, zero
> +	csrw	fcsr, 0
> +#endif /* CONFIG_FPU */
> +.Lreset_regs_done:
> +	ret
> +END(reset_regs)
> +#endif /* CONFIG_M_MODE */
> +
>  __PAGE_ALIGNED_BSS
>  	/* Empty zero page */
>  	.balign PAGE_SIZE

-- 
Regards,
Atish