Re: [PATCH v3] riscv: thead: Add th.sxstatus CSR emulation

[Alistair Francis]

On Thu, Apr 18, 2024 at 8:55 AM Christoph Müllner
<christoph.muellner@vrull.eu> wrote:
>
> The th.sxstatus CSR can be used to identify available custom extension
> on T-Head CPUs. The CSR is documented here:
>   
> https://github.com/T-head-Semi/thead-extension-spec/blob/master/xtheadsxstatus.adoc
>
> An important property of this patch is, that the th.sxstatus MAEE field
> is not set (indicating that XTheadMae is not available).
> XTheadMae is a memory attribute extension (similar to Svpbmt) which is
> implemented in many T-Head CPUs (C906, C910, etc.) and utilizes bits
> in PTEs that are marked as reserved. QEMU maintainers prefer to not
> implement XTheadMae, so we need give kernels a mechanism to identify
> if XTheadMae is available in a system or not. And this patch introduces
> this mechanism in QEMU in a way that's compatible with real HW
> (i.e., probing the th.sxstatus.MAEE bit).
>
> Further context can be found on the list:
> https://lists.gnu.org/archive/html/qemu-devel/2024-02/msg00775.html
>
> Reviewed-by: LIU Zhiwei <zhiwe_liu@linux.alibaba.com>
> Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
> ---
>  target/riscv/cpu.c       |  1 +
>  target/riscv/cpu.h       |  4 +++
>  target/riscv/csr.c       |  2 +-
>  target/riscv/meson.build |  1 +
>  target/riscv/th_csr.c    | 68 ++++++++++++++++++++++++++++++++++++++++
>  5 files changed, 75 insertions(+), 1 deletion(-)
>  create mode 100644 target/riscv/th_csr.c
>
> diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> index 36e3e5fdaf..b82ba95ae6 100644
> --- a/target/riscv/cpu.c
> +++ b/target/riscv/cpu.c
> @@ -545,6 +545,7 @@ static void rv64_thead_c906_cpu_init(Object *obj)
>      cpu->cfg.mvendorid = THEAD_VENDOR_ID;
>  #ifndef CONFIG_USER_ONLY
>      set_satp_mode_max_supported(cpu, VM_1_10_SV39);
> +    th_register_custom_csrs(cpu);
>  #endif
>
>      /* inherited from parent obj via riscv_cpu_init() */
> diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
> index 3b1a02b944..fd9424c8e9 100644
> --- a/target/riscv/cpu.h
> +++ b/target/riscv/cpu.h
> @@ -818,10 +818,14 @@ extern const bool valid_vm_1_10_32[], 
> valid_vm_1_10_64[];
>
>  void riscv_get_csr_ops(int csrno, riscv_csr_operations *ops);
>  void riscv_set_csr_ops(int csrno, riscv_csr_operations *ops);
> +RISCVException smode(CPURISCVState *env, int csrno);

I don't think we should make this public. Especially not the name `smode()`.

One option is to rename this, something like `riscv_csr_check_smode()` maybe?

The better option is probably just to copy it to the vendor
implementation as it's a pretty simple function.

Alistair

>
>  void riscv_cpu_register_gdb_regs_for_features(CPUState *cs);
>
>  uint8_t satp_mode_max_from_map(uint32_t map);
>  const char *satp_mode_str(uint8_t satp_mode, bool is_32_bit);
>
> +/* Implemented in th_csr.c */
> +void th_register_custom_csrs(RISCVCPU *cpu);
> +
>  #endif /* RISCV_CPU_H */
> diff --git a/target/riscv/csr.c b/target/riscv/csr.c
> index 726096444f..503eeb5f24 100644
> --- a/target/riscv/csr.c
> +++ b/target/riscv/csr.c
> @@ -260,7 +260,7 @@ static RISCVException aia_any32(CPURISCVState *env, int 
> csrno)
>      return any32(env, csrno);
>  }
>
> -static RISCVException smode(CPURISCVState *env, int csrno)
> +RISCVException smode(CPURISCVState *env, int csrno)
>  {
>      if (riscv_has_ext(env, RVS)) {
>          return RISCV_EXCP_NONE;
> diff --git a/target/riscv/meson.build b/target/riscv/meson.build
> index a5e0734e7f..a4bd61e52a 100644
> --- a/target/riscv/meson.build
> +++ b/target/riscv/meson.build
> @@ -33,6 +33,7 @@ riscv_system_ss.add(files(
>    'monitor.c',
>    'machine.c',
>    'pmu.c',
> +  'th_csr.c',
>    'time_helper.c',
>    'riscv-qmp-cmds.c',
>  ))
> diff --git a/target/riscv/th_csr.c b/target/riscv/th_csr.c
> new file mode 100644
> index 0000000000..44e28a9298
> --- /dev/null
> +++ b/target/riscv/th_csr.c
> @@ -0,0 +1,68 @@
> +/*
> + * T-Head-specific CSRs.
> + *
> + * Copyright (c) 2024 VRULL GmbH
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms and conditions of the GNU General Public License,
> + * version 2 or later, as published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope it will be useful, but WITHOUT
> + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> + * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
> + * more details.
> + *
> + * You should have received a copy of the GNU General Public License along 
> with
> + * this program.  If not, see <http://www.gnu.org/licenses/>.
> + */
> +
> +#include "qemu/osdep.h"
> +#include "cpu.h"
> +#include "cpu_vendorid.h"
> +
> +#define CSR_TH_SXSTATUS 0x5c0
> +
> +/* TH_SXSTATUS bits */
> +#define TH_SXSTATUS_UCME        BIT(16)
> +#define TH_SXSTATUS_MAEE        BIT(21)
> +#define TH_SXSTATUS_THEADISAEE  BIT(22)
> +
> +typedef struct {
> +    int csrno;
> +    int (*insertion_test)(RISCVCPU *cpu);
> +    riscv_csr_operations csr_ops;
> +} riscv_csr;
> +
> +static int test_thead_mvendorid(RISCVCPU *cpu)
> +{
> +    if (cpu->cfg.mvendorid != THEAD_VENDOR_ID)
> +        return -1;
> +
> +    return 0;
> +}
> +
> +static RISCVException read_th_sxstatus(CPURISCVState *env, int csrno,
> +                                       target_ulong *val)
> +{
> +    /* We don't set MAEE here, because QEMU does not implement MAEE. */
> +    *val = TH_SXSTATUS_UCME | TH_SXSTATUS_THEADISAEE;
> +    return RISCV_EXCP_NONE;
> +}
> +
> +static riscv_csr th_csr_list[] = {
> +    {
> +        .csrno = CSR_TH_SXSTATUS,
> +        .insertion_test = test_thead_mvendorid,
> +        .csr_ops = { "th.sxstatus", smode, read_th_sxstatus }
> +    }
> +};
> +
> +void th_register_custom_csrs(RISCVCPU *cpu)
> +{
> +    for (size_t i = 0; i < ARRAY_SIZE(th_csr_list); i++) {
> +        int csrno = th_csr_list[i].csrno;
> +        riscv_csr_operations *csr_ops = &th_csr_list[i].csr_ops;
> +        if (!th_csr_list[i].insertion_test(cpu))
> +            riscv_set_csr_ops(csrno, csr_ops);
> +    }
> +}
> --
> 2.44.0
>
>