[RFC PATCH 03/14] RISC-V: paravirt: Implement steal-time support

[Andrew Jones]

On Mon, Apr 17, 2023 at 12:33:51PM +0200, Andrew Jones 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.
> 
> Signed-off-by: Andrew Jones <ajones at ventanamicro.com>
> ---
>  arch/riscv/kernel/paravirt.c | 56 ++++++++++++++++++++++++++++++++++--
>  1 file changed, 53 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/riscv/kernel/paravirt.c b/arch/riscv/kernel/paravirt.c
> index 141dbcc36fa2..5f8d96b919e4 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 +40,65 @@ 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_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_set_steal_time_shmem(unsigned long lo, unsigned long hi,
> +					unsigned long flags)
>  {
> +	struct sbiret ret;
> +
> +	ret = sbi_ecall(SBI_EXT_STA, SBI_EXT_STA_SET_STEAL_TIME_SHMEM,
> +			lo, hi, flags, 0, 0, 0);
> +	if (ret.error) {
> +		if (lo == -1 && hi == -1)
> +			pr_warn("Failed to disable steal-time shmem");
> +		else
> +			pr_warn("Failed to set steal-time shmem");
> +		return -ENOMEM;
> +	}
> +
>  	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_set_steal_time_shmem(lo, hi, 0);
> +}
> +
>  static int pv_time_cpu_down_prepare(unsigned int cpu)
>  {
> -	return 0;
> +	return sbi_sta_set_steal_time_shmem(-1, -1, 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;
> +
> +	do {
> +		sequence = st->sequence;
> +		virt_rmb();
> +		steal = st->steal;
> +		virt_rmb();
> +	} while ((sequence & 1) || (sequence != st->sequence));
> +
> +	return steal;
>  }

So anybody poking around the implementations for other architectures will
see that I shamelessly ripped this off from x86's implementation. However,
looking at it again, I think all the references to the steal-time info
should be wrapped in READ_ONCE(). I'll write a patch for x86/kvm to
add READ_ONCE's and also update this patch for rfc-v2.

Thanks,
drew