linux-kernel - Re: [PATCH] riscv: set pmp configuration if kernel is running in M-mode

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Message-ID: <CAHCEeh+4a0O7tpp4dRXKudc6bmdJct=-H0SrPt=HbOs00T3-Hg@mail.gmail.com>
Date:   Thu, 30 Jan 2020 10:38:43 +0800
From:   Greentime Hu <greentime.hu@...ive.com>
To:     Palmer Dabbelt <palmerdabbelt@...gle.com>
Cc:     Gt <green.hu@...il.com>, greentime@...nel.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] riscv: set pmp configuration if kernel is running in M-mode

On Thu, Jan 30, 2020 at 3:23 AM Palmer Dabbelt <palmerdabbelt@...gle.com> wrote:
>
> On Thu, 09 Jan 2020 03:17:40 GMT (+0000), greentime.hu@...ive.com wrote:
> > When the kernel is running in S-mode, the expectation is that the
> > bootloader or SBI layer will configure the PMP to allow the kernel to
> > access physical memory.  But, when the kernel is running in M-mode and is
> > started with the ELF "loader", there's probably no bootloader or SBI layer
> > involved to configure the PMP.  Thus, we need to configure the PMP
> > ourselves to enable the kernel to access all regions.
> >
> > Signed-off-by: Greentime Hu <greentime.hu@...ive.com>
> > ---
> >  arch/riscv/include/asm/csr.h | 12 ++++++++++++
> >  arch/riscv/kernel/head.S     |  6 ++++++
> >  2 files changed, 18 insertions(+)
> >
> > diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
> > index 0a62d2d68455..0f25e6c4e45c 100644
> > --- a/arch/riscv/include/asm/csr.h
> > +++ b/arch/riscv/include/asm/csr.h
> > @@ -72,6 +72,16 @@
> >  #define EXC_LOAD_PAGE_FAULT  13
> >  #define EXC_STORE_PAGE_FAULT 15
> >
> > +/* PMP configuration */
> > +#define PMP_R                        0x01
> > +#define PMP_W                        0x02
> > +#define PMP_X                        0x04
> > +#define PMP_A                        0x18
> > +#define PMP_A_TOR            0x08
> > +#define PMP_A_NA4            0x10
> > +#define PMP_A_NAPOT          0x18
> > +#define PMP_L                        0x80
> > +
> >  /* symbolic CSR names: */
> >  #define CSR_CYCLE            0xc00
> >  #define CSR_TIME             0xc01
> > @@ -100,6 +110,8 @@
> >  #define CSR_MCAUSE           0x342
> >  #define CSR_MTVAL            0x343
> >  #define CSR_MIP                      0x344
> > +#define CSR_PMPCFG0          0x3a0
> > +#define CSR_PMPADDR0         0x3b0
> >  #define CSR_MHARTID          0xf14
> >
> >  #ifdef CONFIG_RISCV_M_MODE
> > diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> > index 5c8b24bf4e4e..f8f996916c5b 100644
> > --- a/arch/riscv/kernel/head.S
> > +++ b/arch/riscv/kernel/head.S
> > @@ -60,6 +60,12 @@ _start_kernel:
> >       /* Reset all registers except ra, a0, a1 */
> >       call reset_regs
> >
> > +     /* Setup a PMP to permit access to all of memory. */
> > +     li a0, -1
> > +     csrw CSR_PMPADDR0, a0
> > +     li a0, (PMP_A_NAPOT | PMP_R | PMP_W | PMP_X)
> > +     csrw CSR_PMPCFG0, a0
>
> These should be guarded by some sort of #ifdef CONFIG_M_MODE, as they're not
> part of S mode.

Hi Palmer,

This code segment is guarded by CONFIG_RISCV_M_MODE

#ifdef CONFIG_RISCV_M_MODE
        /* flush the instruction cache */
        fence.i

        /* Reset all registers except ra, a0, a1 */
        call reset_regs

        /* Setup a PMP to permit access to all of memory. */
        li a0, -1
        csrw CSR_PMPADDR0, a0
        li a0, (PMP_A_NAPOT | PMP_R | PMP_W | PMP_X)
        csrw CSR_PMPCFG0, a0

        /*
         * The hartid in a0 is expected later on, and we have no firmware
         * to hand it to us.
         */
        csrr a0, CSR_MHARTID
#endif /* CONFIG_RISCV_M_MODE */