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Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

On 10/07/2009 09:29 PM, Jeremy Fitzhardinge wrote:

   I'm a bit worried about the kernel playing with the hypervisor's
version field.
For Xen I explicitly made it not a problem by adding the notion of a
secondary pvclock_vcpu_time_info structure which is updated by copying,
aside from the version number which is updated as-is.

When do you copy?

I'd rather have a single copy for guest and host.

As far as I can tell it isn't a problem for KVM either.  The guest
version number is atomic with respect to preemption by the hypervisor so
there's no scope for racing.  (The ABI already guarantees that the
pvclock structures are never updated cross-cpu.)

It ultimately doesn't matter what the version number is so long as it
changes when the parameters are updated, and version numbers can't be
reused within a window where things get confused.

If the hypervisor does a pvclock->version = somethingelse->version++ then the guest may get confused. But I understand you have a guest-private ->version?

It's better to introduce yet a new version for the kernel, and check
Two version numbers are awkward to read atomically at least on 32-bit.
And I don't think its necessary.

No need to read them atomically.

cpu1 = vgetcpu()
hver1 = pvclock[cpu1].hver
kver1 = pvclock[cpu1].kver
tsc = rdtsc
/* multipication magic with pvclock[cpu1]*/
cpu2 = vgetcpu()
hver2 = pvclock[cpu2].hver
kver2 = pvclock[cpu2].kver
valid = cpu1 == cpu2 && hver1 == hver2 && kver1 == kver2

It's sufficient to increment a version counter on thread migration, no
need to do it on context switch.

That's true; switch_out is a pessimistic approximation of that.  But is
there a convenient hook to test for migration?

I'd guess not but it's probably easy to add one in the migration thread.

Do not meddle in the internals of kernels, for they are subtle and quick to 

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