[Xen-devel] Re: hvm vpt lock strangeness

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

Locking is required to prevent multiple concurrent access to a single
periodic timer. This is actually implemented at vcpu granularity to avoid
lock juggling in functions which walk lists of timers. Since those lists are
held per-vcpu, the convenient locking scope is the vcpu.

 -- Keir

On 23/5/08 21:27, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> Noticed this a few days ago while working on the hvm-guest-time
> on xen-system-time patch, but forgot about it...
> 
> Throughout xen/arch/x86/hvm/vpt.c, there are uses of a spinlock
> called tm_lock.  But it appears that this spinlock is declared
> and used as part of a per-vcpu data structure.  So is this
> somehow protecting against vcpu re-entrancy (didn't think that
> could happen) or is it supposed to be locking out one vcpu
> against another (and not doing the job because each vcpu has
> a separate lock)?  Or am I misunderstanding something entirely?
> 
> If this should be a domain-wide lock, I'll spin a patch.
> (And it might explain some of the weirder time problems?)
> 
> Thanks,
> Dan
> 
> ===================================
> Thanks... for the memory
> I really could use more / My throughput's on the floor
> The balloon is flat / My swap disk's fat / I've OOM's in store
> Overcommitted so much
> (with apologies to the late great Bob Hope)



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