Re: [Xen-devel] [PATCH 3/3] VMX: Remove the vcpu from the per-cpu blocking list after domain termination

["Wu, Feng" <feng.wu@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Monday, May 23, 2016 5:08 PM
> To: Wu, Feng <feng.wu@xxxxxxxxx>
> Cc: andrew.cooper3@xxxxxxxxxx; dario.faggioli@xxxxxxxxxx;
> george.dunlap@xxxxxxxxxxxxx; Tian, Kevin <kevin.tian@xxxxxxxxx>; xen-
> devel@xxxxxxxxxxxxx; konrad.wilk@xxxxxxxxxx; keir@xxxxxxx
> Subject: RE: [PATCH 3/3] VMX: Remove the vcpu from the per-cpu blocking list
> after domain termination
> 
> >>> On 23.05.16 at 07:48, <feng.wu@xxxxxxxxx> wrote:
> >> From: Tian, Kevin
> >> Sent: Monday, May 23, 2016 1:19 PM
> >> > From: Wu, Feng
> >> > Sent: Friday, May 20, 2016 4:54 PM
> >> > --- a/xen/arch/x86/hvm/vmx/vmx.c
> >> > +++ b/xen/arch/x86/hvm/vmx/vmx.c
> >> > @@ -248,6 +248,36 @@ void vmx_pi_hooks_deassign(struct domain *d)
> >> >      d->arch.hvm_domain.vmx.pi_switch_to = NULL;
> >> >  }
> >> >
> >> > +static void vmx_pi_blocking_list_cleanup(struct domain *d)
> >>
> >> Is it more natural to move such cleanup under vcpu destroy?
> >
> > Yes, indeed it is more natural to add this function when vcpu is destroyed,
> > however, the reason I add it here is I still have some concerns about the
> > timing.
> > When the VM is destroyed, here is the calling path:
> >
> > - vmx_pi_hooks_deassign() -->
> > ......
> > - vmx_vcpu_destroy() -->
> > ......
> > - vmx_domain_destroy()
> > ......
> >
> > As I replied in the previous mail, when we remove the vcpus from the
> > blocking
> > list, there might be some _in-flight_ call to the hooks, so I put the 
> > cleanup
> > code in the vmx_domain_destroy(), which is a bit more far from
> > vmx_pi_hooks_deassign,
> > and hence safer. If you have any other good ideas, I am all ears!:)
> 
> Well, either there is a possible race (then moving the addition
> later just reduces the chances, but doesn't eliminate it), or there
> isn't (in which case Kevin's suggestion should probably be followed).

Yes, I agree, adding the cleanup code in domain destroy other than
vcpu destroy point just reduces the risk, but not eliminate. So far I don't
get a perfect solution to solve this possible race condition.

Thanks,
Feng

> 
> Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel