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

Avi Kivity
2009-10-07T20:10:57Z

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
>> both.
>>
> 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 panic.