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

Andrew Jones
2023-04-19T08:42:31Z

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@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