Re: [Xen-devel] [PATCH 5/8] x86/vm-event/monitor: don't compromise monitor_write_data on domain cleanup

[Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx>]

On 7/4/2016 5:17 PM, Jan Beulich wrote:
> > > > On 04.07.16 at 15:50, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> > On 07/04/16 16:11, Jan Beulich wrote:
> > > > > > On 04.07.16 at 15:03, <czuzu@xxxxxxxxxxxxxxx> wrote:
> > > > On 7/4/2016 3:47 PM, Jan Beulich wrote:
> > > > > > > > On 30.06.16 at 20:45, <czuzu@xxxxxxxxxxxxxxx> wrote:
> > > > > > The arch_vm_event structure is dynamically allocated and freed @
> > > > > > vm_event_cleanup_domain. This cleanup is triggered e.g. when the toolstack
> > user
> > > > > > disables domain monitoring (xc_monitor_disable), which in turn effectively
> > > > > > discards any information that was in arch_vm_event.write_data.
> > > > > Isn't that rather a toolstack user bug, not warranting a relatively
> > > > > extensive (even if mostly mechanical) hypervisor change like this
> > > > > one? Sane monitor behavior, after all, is required anyway for the
> > > > > monitored guest to survive.
> > > > Sorry but could you please rephrase this, I don't quite understand what
> > > > you're saying.
> > > > The write_data field in arch_vm_event should _not ever_ be invalidated
> > > > as a direct result of a toolstack user's action.
> > > The monitoring app can cause all kinds of problems to the guest it
> > > monitors. Why would this specific one need taking care of in the
> > > hypervisor, instead of demanding that the app not disable monitoring
> > > at the wrong time?
> > I'm not sure there's a right time here. The problem is that, if I
> > understand this correctly, a race is possible between the moment the
> > userspace application responds to the vm_event _and_ call
> > xc_monitor_disable() and the time hvm_do_resume() gets called.
> It's that _and_ in your reply that I put under question, but I admit
> that questioning may be due to my limited (or should I say non-
> existent) knowledge on the user space parts here: Why would the
> app be _required_ to "responds to the vm_event _and_ call
> xc_monitor_disable()", rather than delaying the latter for long
> enough?
>
> Jan
Because it's a limitation that can easily be avoided and also a rule 
which the user has to keep in mind (why should we impose this effort on 
the toolstack user if not necessary?).
Corneliu.

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