Re: [PATCH 12/15] riscv: clear the instruction cache and all registers when booting

From: Anup Patel <anup@brainfault.org>
To: Christoph Hellwig <hch@lst.de>
Cc: "linux-kernel@vger.kernel.org List"
	<linux-kernel@vger.kernel.org>,
	Damien Le Moal <damien.lemoal@wdc.com>,
	Palmer Dabbelt <palmer@sifive.com>,
	linux-riscv <linux-riscv@lists.infradead.org>,
	Paul Walmsley <paul.walmsley@sifive.com>
Subject: Re: [PATCH 12/15] riscv: clear the instruction cache and all registers when booting
Date: Fri, 18 Oct 2019 08:35:14 +0530	[thread overview]
Message-ID: <CAAhSdy2mjtH68Ce+h4u3pa79pB-Vs_iu1zfRRcGoaYr=r6xnQw@mail.gmail.com> (raw)
In-Reply-To: <20191017173743.5430-13-hch@lst.de>

On Thu, Oct 17, 2019 at 11:08 PM Christoph Hellwig <hch@lst.de> 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.  Vaguely based on the concepts from opensbi.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  arch/riscv/include/asm/csr.h |  1 +
>  arch/riscv/kernel/head.S     | 88 +++++++++++++++++++++++++++++++++++-
>  2 files changed, 88 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
> index d0b5113e1a54..ee0101278608 100644
> --- a/arch/riscv/include/asm/csr.h
> +++ b/arch/riscv/include/asm/csr.h
> @@ -83,6 +83,7 @@
>  /* symbolic CSR names: */
>  #define CSR_MHARTID            0xf14
>  #define CSR_MSTATUS            0x300
> +#define CSR_MISA               0x301
>  #define CSR_MIE                        0x304
>  #define CSR_MTVEC              0x305
>  #define CSR_MSCRATCH           0x340
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index 583784cb3a32..25867b99cc95 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>
>  #include <asm/image.h>
>
>  __INIT
> @@ -51,12 +52,18 @@ _start_kernel:
>         csrw CSR_XIP, zero
>
>  #ifdef CONFIG_RISCV_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.
>          */
>         csrr a0, CSR_MHARTID
> -#endif
> +#endif /* CONFIG_RISCV_M_MODE */
>
>         /* Load the global pointer */
>  .option push
> @@ -203,6 +210,85 @@ relocate:
>         j .Lsecondary_park
>  END(_start)
>
> +#ifdef CONFIG_RISCV_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, CSR_MISA
> +       andi    t0, t0, (COMPAT_HWCAP_ISA_F | COMPAT_HWCAP_ISA_D)
> +       bnez    t0, .Lreset_regs_done
> +
> +       li      t1, SR_FS
> +       csrs    CSR_XSTATUS, 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
> +       /* note that the caller must clear SR_FS */
> +#endif /* CONFIG_FPU */
> +.Lreset_regs_done:
> +       ret
> +END(reset_regs)
> +#endif /* CONFIG_RISCV_M_MODE */
> +
>  __PAGE_ALIGNED_BSS
>         /* Empty zero page */
>         .balign PAGE_SIZE
> --
> 2.20.1
>
>
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LGTM.

Reviewed-by: Anup Patel <anup@brainfault.org>

Regards,
Anup

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