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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Dec 22, 2014 at 04:39:57PM -0800, 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.
> 
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> ---
>  arch/x86/vdso/vclock_gettime.c | 82 ++++++++++++++++++++++++------------------
>  1 file changed, 47 insertions(+), 35 deletions(-)
> 
> diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
> index 9793322751e0..f2e0396d5629 100644
> --- 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.
>  	 */

Can Xen guarantee that ?

> -	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;
> +	}

This check must be performed after reading a stable pvti.

> +
> +	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);

The following is possible:

1) State: all pvtis marked as PVCLOCK_TSC_STABLE_BIT.
1) Update request for all vcpus, for a TSC_STABLE_BIT -> ~TSC_STABLE_BIT
transition.
2) vCPU-1 updates its pvti with new values.
3) vCPU-0 still has not updated its pvti with new values.
4) vCPU-1 VM-enters, uses vCPU-0 values, even though it has been
notified of a TSC_STABLE_BIT -> ~TSC_STABLE_BIT transition.

The update is not actually atomic across all vCPUs, its atomic in
the sense of not allowing visibility of distinct
system_timestamp/tsc_timestamp values.