Re: [Xen-devel] [RFC 2/2] x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader

David Vrabel
2014-12-23T10:28:26Z

On 23/12/14 00:39, Andy Lutomirski wrote:
> The pvclock vdso code was too abstracted to understand easily and
> excessively paranoid. Simplify it for a huge speedup.
>
> This opens the door for additional simplifications, as the vdso no
> longer accesses the pvti for any vcpu other than vcpu 0.
>
> Before, vclock_gettime using kvm-clock took about 64ns on my machine.
> With this change, it takes 19ns, which is almost as fast as the pure TSC
> implementation.

This sounds plausible but I'm not going to be able to give it a detailed
look until the new year.

David

> --- a/arch/x86/vdso/vclock_gettime.c
> +++ b/arch/x86/vdso/vclock_gettime.c
> @@ -78,47 +78,59 @@ static notrace const struct pvclock_vsyscall_time_info *get_pvti(int cpu)
>
> static notrace cycle_t vread_pvclock(int *mode)
> {
> - const struct pvclock_vsyscall_time_info *pvti;
> + const struct pvclock_vcpu_time_info *pvti = &get_pvti(0)->pvti;
> cycle_t ret;
> - u64 last;
> - u32 version;
> - u8 flags;
> - unsigned cpu, cpu1;
> -
> + u64 tsc, pvti_tsc;
> + u64 last, delta, pvti_system_time;
> + u32 version, pvti_tsc_to_system_mul, pvti_tsc_shift;
>
> /*
> - * Note: hypervisor must guarantee that:
> - * 1. cpu ID number maps 1:1 to per-CPU pvclock time info.
> - * 2. that per-CPU pvclock time info is updated if the
> - * underlying CPU changes.
> - * 3. that version is increased whenever underlying CPU
> - * changes.
> + * Note: The kernel and hypervisor must guarantee that cpu ID
> + * number maps 1:1 to per-CPU pvclock time info.
> + *
> + * Because the hypervisor is entirely unaware of guest userspace
> + * preemption, it cannot guarantee that per-CPU pvclock time
> + * info is updated if the underlying CPU changes or that that
> + * version is increased whenever underlying CPU changes.
> + *
> + * On KVM, we are guaranteed that pvti updates for any vCPU are
> + * atomic as seen by *all* vCPUs. This is an even stronger
> + * guarantee than we get with a normal seqlock.
> *
> + * On Xen, we don't appear to have that guarantee, but Xen still
> + * supplies a valid seqlock using the version field.
> +
> + * We only do pvclock vdso timing at all if
> + * PVCLOCK_TSC_STABLE_BIT is set, and we interpret that bit to
> + * mean that all vCPUs have matching pvti and that the TSC is
> + * synced, so we can just look at vCPU 0's pvti.
> */
> - do {
> - cpu = __getcpu() & VGETCPU_CPU_MASK;
> - /* TODO: We can put vcpu id into higher bits of pvti.version.
> - * This will save a couple of cycles by getting rid of
> - * __getcpu() calls (Gleb).
> - */
> -
> - pvti = get_pvti(cpu);
> -
> - version = __pvclock_read_cycles(&pvti->pvti, &ret, &flags);
> -
> - /*
> - * Test we're still on the cpu as well as the version.
> - * We could have been migrated just after the first
> - * vgetcpu but before fetching the version, so we
> - * wouldn't notice a version change.
> - */
> - cpu1 = __getcpu() & VGETCPU_CPU_MASK;
> - } while (unlikely(cpu != cpu1 ||
> - (pvti->pvti.version & 1) ||
> - pvti->pvti.version != version));
> -
> - if (unlikely(!(flags & PVCLOCK_TSC_STABLE_BIT)))
> +
> + if (unlikely(!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))) {
> *mode = VCLOCK_NONE;
> + return 0;
> + }
> +
> + do {
> + version = pvti->version;
> +
> + /* This is also a read barrier, so we'll read version first. */
> + rdtsc_barrier();
> + tsc = __native_read_tsc();
> +
> + pvti_tsc_to_system_mul = pvti->tsc_to_system_mul;
> + pvti_tsc_shift = pvti->tsc_shift;
> + pvti_system_time = pvti->system_time;
> + pvti_tsc = pvti->tsc_timestamp;
> +
> + /* Make sure that the version double-check is last. */
> + smp_rmb();
> + } while (unlikely((version & 1) || version != pvti->version));
> +
> + delta = tsc - pvti_tsc;
> + ret = pvti_system_time +
> + pvclock_scale_delta(delta, pvti_tsc_to_system_mul,
> + pvti_tsc_shift);
>
> /* refer to tsc.c read_tsc() comment for rationale */
> last = gtod->cycle_last;
>