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

[Konrad Rzeszutek Wilk]

On Mon, Jan 05, 2015 at 10:56:07AM -0800, Andy Lutomirski wrote:
> On Mon, Jan 5, 2015 at 7:25 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> > 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@xxxxxxxxxxxxxx>
> >> ---
> >>  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 ?
> 
> I think so, vacuously.  Xen doesn't seem to set PVCLOCK_TSC_STABLE_BIT
> at all.  I have no idea going forward, though.
> 
> Xen people?

The person who would know of the top of his head is Dan Magenheimer, who
is now enjoy retirement :-(

I will have to dig in the code to answer this - that will take a bit of time
sadly (I am sick this week).
> 
> >
> >> -     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.
> >
> 
> We can even read it in the middle, guarded by the version checks.
> I'll do that for v2.
> 
> >> +
> >> +     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.
> >
> 
> Hmm.  In step 4, is there a guarantee that vCPU-0 won't VM-enter until
> it gets marked unstable?  Otherwise the vdso could could just as
> easily be called from vCPU-1, migrated to vCPU-0, read the data
> complete with stale stable bit, and get migrated back to vCPU-1.
> 
> But I thought that KVM currently froze all vCPUs when updating pvti
> for any of them.  How can this happen?  I admit I don't really
> understand the update request code.
> 
> --Andy
> 
> -- 
> Andy Lutomirski
> AMA Capital Management, LLC
> 
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