[PATCH] lib: sbi: Flush cache entries after writing PMP CSRs

[Anup Patel]

On Thu, Sep 4, 2025 at 4:26 PM <cp0613 at linux.alibaba.com> wrote:
>
> From: Chen Pei <cp0613 at linux.alibaba.com>
>
> As the privileged specification states, after writing to the PMP CSRs,
> a SFENCE.VMA or HFENCE.GVMA instruction should be executed with rs1=x0
> and rs2=x0 to flush all address translation cache entries.
>
> The original implementation does not cover all possible cases. For
> example, the sbi_hart_map_saddr function calls pmp_set but does not
> execute the SFENCE.VMA instruction. This patch covers sbi_hart_map_saddr
> and sbi_hart_unmap_saddr, ensuring that dbtr, sse and other modules can
> safely update pmpcfg.

The temporary mapping created using sbi_hart_map/unmap_saddr() is
only accessed by M-mode and does not impact the S-mode TLB
entries in anyway.

Further, flushing TLB entries in sbi_hart_map/unmap_saddr() also
slows-down many SBI calls which use shared memory.

NACK to this patch.

Regards,
Anup

>
> Signed-off-by: Chen Pei <cp0613 at linux.alibaba.com>
> ---
>  lib/sbi/sbi_hart.c | 54 ++++++++++++++++++++++++++++------------------
>  1 file changed, 33 insertions(+), 21 deletions(-)
>
> diff --git a/lib/sbi/sbi_hart.c b/lib/sbi/sbi_hart.c
> index 6a2d7d6..33bba38 100644
> --- a/lib/sbi/sbi_hart.c
> +++ b/lib/sbi/sbi_hart.c
> @@ -364,6 +364,30 @@ static unsigned int sbi_hart_get_smepmp_flags(struct sbi_scratch *scratch,
>         return pmp_flags;
>  }
>
> +static void pmp_flush(void)
> +{
> +       /*
> +        * As per section 3.7.2 of privileged specification v1.12,
> +        * virtual address translations can be speculatively performed
> +        * (even before actual access). These, along with PMP traslations,
> +        * can be cached. This can pose a problem with CPU hotplug
> +        * and non-retentive suspend scenario because PMP states are
> +        * not preserved.
> +        * It is advisable to flush the caching structures under such
> +        * conditions.
> +        */
> +       if (misa_extension('S')) {
> +               __asm__ __volatile__("sfence.vma");
> +
> +               /*
> +                * If hypervisor mode is supported, flush caching
> +                * structures in guest mode too.
> +                */
> +               if (misa_extension('H'))
> +                       __sbi_hfence_gvma_all();
> +       }
> +}
> +
>  static void sbi_hart_smepmp_set(struct sbi_scratch *scratch,
>                                 struct sbi_domain *dom,
>                                 struct sbi_domain_memregion *reg,
> @@ -543,18 +567,25 @@ int sbi_hart_map_saddr(unsigned long addr, unsigned long size)
>                              pmp_flags, base, order);
>         pmp_set(SBI_SMEPMP_RESV_ENTRY, pmp_flags, base, order);
>
> +       pmp_flush();
> +
>         return SBI_OK;
>  }
>
>  int sbi_hart_unmap_saddr(void)
>  {
> +       int rc;
>         struct sbi_scratch *scratch = sbi_scratch_thishart_ptr();
>
>         if (!sbi_hart_has_extension(scratch, SBI_HART_EXT_SMEPMP))
>                 return SBI_OK;
>
>         sbi_platform_pmp_disable(sbi_platform_ptr(scratch), SBI_SMEPMP_RESV_ENTRY);
> -       return pmp_disable(SBI_SMEPMP_RESV_ENTRY);
> +       rc = pmp_disable(SBI_SMEPMP_RESV_ENTRY);
> +
> +       pmp_flush();
> +
> +       return rc;
>  }
>
>  int sbi_hart_pmp_configure(struct sbi_scratch *scratch)
> @@ -578,26 +609,7 @@ int sbi_hart_pmp_configure(struct sbi_scratch *scratch)
>                 rc = sbi_hart_oldpmp_configure(scratch, pmp_count,
>                                                 pmp_log2gran, pmp_addr_max);
>
> -       /*
> -        * As per section 3.7.2 of privileged specification v1.12,
> -        * virtual address translations can be speculatively performed
> -        * (even before actual access). These, along with PMP traslations,
> -        * can be cached. This can pose a problem with CPU hotplug
> -        * and non-retentive suspend scenario because PMP states are
> -        * not preserved.
> -        * It is advisable to flush the caching structures under such
> -        * conditions.
> -        */
> -       if (misa_extension('S')) {
> -               __asm__ __volatile__("sfence.vma");
> -
> -               /*
> -                * If hypervisor mode is supported, flush caching
> -                * structures in guest mode too.
> -                */
> -               if (misa_extension('H'))
> -                       __sbi_hfence_gvma_all();
> -       }
> +       pmp_flush();
>
>         return rc;
>  }
> --
> 2.49.0
>
>
> --
> opensbi mailing list
> opensbi at lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/opensbi