[PATCH v2 03/13] RISC-V: paravirt: Implement steal-time support

[Anup Patel]

On Thu, Dec 14, 2023 at 3:45 PM Andrew Jones <ajones at ventanamicro.com> wrote:
>
> When the SBI STA extension exists we can use it to implement
> paravirt steal-time support. Fill in the empty pv-time functions
> with an SBI STA implementation and add the Kconfig knobs allowing
> it to be enabled.
>
> Signed-off-by: Andrew Jones <ajones at ventanamicro.com>
> ---
>  arch/riscv/Kconfig           | 19 ++++++++++
>  arch/riscv/kernel/paravirt.c | 67 ++++++++++++++++++++++++++++++++++--
>  2 files changed, 83 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 95a2a06acc6a..b99fd8129edf 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -724,6 +724,25 @@ config COMPAT
>
>           If you want to execute 32-bit userspace applications, say Y.
>
> +config PARAVIRT
> +       bool "Enable paravirtualization code"
> +       depends on RISCV_SBI
> +       help
> +         This changes the kernel so it can modify itself when it is run
> +         under a hypervisor, potentially improving performance significantly
> +         over full virtualization.
> +
> +config PARAVIRT_TIME_ACCOUNTING
> +       bool "Paravirtual steal time accounting"
> +       depends on PARAVIRT
> +       help
> +         Select this option to enable fine granularity task steal time
> +         accounting. Time spent executing other tasks in parallel with
> +         the current vCPU is discounted from the vCPU power. To account for
> +         that, there can be a small performance impact.
> +
> +         If in doubt, say N here.
> +
>  config RELOCATABLE
>         bool "Build a relocatable kernel"
>         depends on MMU && 64BIT && !XIP_KERNEL
> diff --git a/arch/riscv/kernel/paravirt.c b/arch/riscv/kernel/paravirt.c
> index 141dbcc36fa2..b09dfd81bcd2 100644
> --- a/arch/riscv/kernel/paravirt.c
> +++ b/arch/riscv/kernel/paravirt.c
> @@ -6,12 +6,21 @@
>  #define pr_fmt(fmt) "riscv-pv: " fmt
>
>  #include <linux/cpuhotplug.h>
> +#include <linux/compiler.h>
> +#include <linux/errno.h>
>  #include <linux/init.h>
>  #include <linux/jump_label.h>
> +#include <linux/kconfig.h>
> +#include <linux/kernel.h>
> +#include <linux/percpu-defs.h>
>  #include <linux/printk.h>
>  #include <linux/static_call.h>
>  #include <linux/types.h>
>
> +#include <asm/barrier.h>
> +#include <asm/page.h>
> +#include <asm/sbi.h>
> +
>  struct static_key paravirt_steal_enabled;
>  struct static_key paravirt_steal_rq_enabled;
>
> @@ -31,24 +40,76 @@ static int __init parse_no_stealacc(char *arg)
>
>  early_param("no-steal-acc", parse_no_stealacc);
>
> +DEFINE_PER_CPU(struct sbi_sta_struct, steal_time) __aligned(64);
> +
>  static bool __init has_pv_steal_clock(void)
>  {
> +       if (sbi_spec_version >= sbi_mk_version(2, 0) &&
> +           sbi_probe_extension(SBI_EXT_STA) > 0) {
> +               pr_info("SBI STA extension detected\n");
> +               return true;
> +       }
> +
>         return false;
>  }
>
> -static int pv_time_cpu_online(unsigned int cpu)
> +static int sbi_sta_steal_time_set_shmem(unsigned long lo, unsigned long hi,
> +                                       unsigned long flags)
>  {
> +       struct sbiret ret;
> +
> +       ret = sbi_ecall(SBI_EXT_STA, SBI_EXT_STA_STEAL_TIME_SET_SHMEM,
> +                       lo, hi, flags, 0, 0, 0);
> +       if (ret.error) {
> +               if (lo == SBI_STA_SHMEM_DISABLE && hi == SBI_STA_SHMEM_DISABLE)
> +                       pr_warn("Failed to disable steal-time shmem");
> +               else
> +                       pr_warn("Failed to set steal-time shmem");
> +               return sbi_err_map_linux_errno(ret.error);
> +       }
> +
>         return 0;
>  }
>
> +static int pv_time_cpu_online(unsigned int cpu)
> +{
> +       struct sbi_sta_struct *st = this_cpu_ptr(&steal_time);
> +       phys_addr_t pa = __pa(st);
> +       unsigned long lo = (unsigned long)pa;
> +       unsigned long hi = IS_ENABLED(CONFIG_32BIT) ? upper_32_bits((u64)pa) : 0;
> +
> +       return sbi_sta_steal_time_set_shmem(lo, hi, 0);
> +}
> +
>  static int pv_time_cpu_down_prepare(unsigned int cpu)
>  {
> -       return 0;
> +       return sbi_sta_steal_time_set_shmem(SBI_STA_SHMEM_DISABLE,
> +                                           SBI_STA_SHMEM_DISABLE, 0);
>  }
>
>  static u64 pv_time_steal_clock(int cpu)
>  {
> -       return 0;
> +       struct sbi_sta_struct *st = per_cpu_ptr(&steal_time, cpu);
> +       u32 sequence;
> +       u64 steal;
> +
> +       if (IS_ENABLED(CONFIG_32BIT)) {
> +               /*
> +                * Check the sequence field before and after reading the steal
> +                * field. Repeat the read if it is different or odd.
> +                */
> +               do {
> +                       sequence = READ_ONCE(st->sequence);
> +                       virt_rmb();
> +                       steal = READ_ONCE(st->steal);
> +                       virt_rmb();
> +               } while ((le32_to_cpu(sequence) & 1) ||
> +                        sequence != READ_ONCE(st->sequence));

Actually, we should be doing this sequence for both RV64 and RV32
because for RV64 the steal time value is valid only when sequence is
an even number.

> +       } else {
> +               steal = READ_ONCE(st->steal);
> +       }
> +
> +       return le64_to_cpu(steal);
>  }
>
>  int __init pv_time_init(void)
> --
> 2.43.0
>

Regards,
Anup