x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader

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 45ns on my
machine. With this change, it takes 29ns, which is almost as
fast as the pure TSC implementation.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/6b51dcc41f1b101f963945c5ec7093d72bdac429.1449702533.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
index ca94fa6492516b0b1d669b6f52b09e968e3ae5cf..c325ba1bdddf4afbccc86a177dd57ce495412a03 100644 (file)
--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -78,47 +78,58 @@ 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. */
+               tsc = rdtsc_ordered();
+
+               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;