Re: [PATCH] target/riscv: Support SW update of PTE A/D bits and Ssptwad extension

[Anup Patel]

+Atish

On Mon, Jul 18, 2022 at 9:23 AM Jim Shu <jim.shu@sifive.com> wrote:
>
> RISC-V priv spec v1.12 permits 2 PTE-update schemes of A/D-bit
> (Access/Dirty bit): HW update or SW update. RISC-V profile defines the
> extension name 'Ssptwad' for HW update to PTE A/D bits.
> https://github.com/riscv/riscv-profiles/blob/main/profiles.adoc

The Ssptwad (even though used by profiles) is not a well defined RISC-V
ISA extension. Rather, Ssptwad is just a name used in profiles to represent
an optional feature.

In fact, since it is not a well-defined ISA extension, QEMU cannot include
Ssptwad in the ISA string as well.

I think for such optionalities which are not well-defined ISA extensions,
QEMU should treat it differently.

Regards,
Anup

>
> Current QEMU RISC-V implements HW update scheme, so this commit
> introduces SW update scheme to QEMU and uses the 'Ssptwad' extension
> as the CPU option to select 2 PTE-update schemes. QEMU RISC-V CPU still
> uses HW update scheme (ext_ssptwad=true) by default to keep the backward
> compatibility.
>
> SW update scheme implemention is based on priv spec v1.12:
> "When a virtual page is accessed and the A bit is clear, or is written
> and the D bit is clear, a page-fault exception (corresponding to the
> original access type) is raised."
>
> Signed-off-by: Jim Shu <jim.shu@sifive.com>
> Reviewed-by: Frank Chang <frank.chang@sifive.com>
> ---
>  target/riscv/cpu.c        | 2 ++
>  target/riscv/cpu.h        | 1 +
>  target/riscv/cpu_helper.c | 9 +++++++++
>  3 files changed, 12 insertions(+)
>
> diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> index 1bb3973806..1d38c1c1d2 100644
> --- a/target/riscv/cpu.c
> +++ b/target/riscv/cpu.c
> @@ -857,6 +857,7 @@ static void riscv_cpu_init(Object *obj)
>
>      cpu->cfg.ext_ifencei = true;
>      cpu->cfg.ext_icsr = true;
> +    cpu->cfg.ext_ssptwad = true;
>      cpu->cfg.mmu = true;
>      cpu->cfg.pmp = true;
>
> @@ -900,6 +901,7 @@ static Property riscv_cpu_extensions[] = {
>      DEFINE_PROP_BOOL("svinval", RISCVCPU, cfg.ext_svinval, false),
>      DEFINE_PROP_BOOL("svnapot", RISCVCPU, cfg.ext_svnapot, false),
>      DEFINE_PROP_BOOL("svpbmt", RISCVCPU, cfg.ext_svpbmt, false),
> +    DEFINE_PROP_BOOL("ssptwad", RISCVCPU, cfg.ext_ssptwad, true),
>
>      DEFINE_PROP_BOOL("zba", RISCVCPU, cfg.ext_zba, true),
>      DEFINE_PROP_BOOL("zbb", RISCVCPU, cfg.ext_zbb, true),
> diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
> index 5c7acc055a..2eee59af98 100644
> --- a/target/riscv/cpu.h
> +++ b/target/riscv/cpu.h
> @@ -433,6 +433,7 @@ struct RISCVCPUConfig {
>      bool ext_zve32f;
>      bool ext_zve64f;
>      bool ext_zmmul;
> +    bool ext_ssptwad;
>      bool rvv_ta_all_1s;
>
>      uint32_t mvendorid;
> diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
> index 59b3680b1b..a8607c0d7b 100644
> --- a/target/riscv/cpu_helper.c
> +++ b/target/riscv/cpu_helper.c
> @@ -981,6 +981,15 @@ restart:
>
>              /* Page table updates need to be atomic with MTTCG enabled */
>              if (updated_pte != pte) {
> +                if (!cpu->cfg.ext_ssptwad) {
> +                    /*
> +                     * If A/D bits are managed by SW, HW just raises the
> +                     * page fault exception corresponding to the original
> +                     * access type when A/D bits need to be updated.
> +                     */
> +                    return TRANSLATE_FAIL;
> +                }
> +
>                  /*
>                   * - if accessed or dirty bits need updating, and the PTE is
>                   *   in RAM, then we do so atomically with a compare and 
> swap.
> --
> 2.17.1
>
>