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

On 23/12/2014 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

Xen guests don't use any of this at the moment, and I don't think this change would prevent us from using it in the future, so:

Acked-by: David Vrabel <david.vrabel@xxxxxxxxxx>

But see some additional comments below.

--- 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;

Xen updates pvti when scheduling a VCPU. Using 0 here requires that VCPU 0 has been recently scheduled by Xen. Perhaps using the current CPU here would be better? It doesn't matter if the task is subsequently moved to a different CPU before using pvti.

+        * 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.

I think this is a much stronger requirement than you actually need.

You only require:

- the system time (pvti->system_time) for all pvti's is synchronized; and
- TSC is synchronized; and
- the pvti has been updated sufficiently recently (so the error in the result is within acceptable margins).

Can you add documentation to arch/x86/include/asm/pvclock-abi.h to describe what properties PVCLOCK_TSC_STABLE_BIT guarantees?


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