Messages in this thread | | | Date | Thu, 08 Jan 2015 12:51:27 +0000 | From | David Vrabel <> | Subject | Re: [Xen-devel] [RFC 2/2] x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader |
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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 > implementation.
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@citrix.com>
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?
David
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