[PATCH v4 03/13] RISC-V: paravirt: Implement steal-time support

[Palmer Dabbelt]

On Wed, 20 Dec 2023 08:00:16 PST (-0800), 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.
>
> Reviewed-by: Atish Patra <atishp at rivosinc.com>
> Reviewed-by: Anup Patel <anup at brainfault.org>
> Signed-off-by: Andrew Jones <ajones at ventanamicro.com>
> ---
>  arch/riscv/Kconfig           | 19 +++++++++++
>  arch/riscv/kernel/paravirt.c | 63 ++++++++++++++++++++++++++++++++++--
>  2 files changed, 79 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 080c95d2a295..574733b2bbea 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 +40,72 @@ 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;
> +
> +	/*
> +	 * 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));
> +
> +	return le64_to_cpu(steal);
>  }
>
>  int __init pv_time_init(void)

Acked-by: Palmer Dabbelt <palmer at rivosinc.com>