Re: [Xen-devel] [PATCH v8 2/7] VMX: Properly handle pi when all the assigned devices are removed

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

> -----Original Message-----
> From: Tian, Kevin
> Sent: Friday, November 18, 2016 12:59 PM
> To: Wu, Feng <feng.wu@xxxxxxxxx>; xen-devel@xxxxxxxxxxxxx
> Cc: jbeulich@xxxxxxxx; andrew.cooper3@xxxxxxxxxx;
> george.dunlap@xxxxxxxxxxxxx; dario.faggioli@xxxxxxxxxx
> Subject: RE: [PATCH v8 2/7] VMX: Properly handle pi when all the assigned
> devices are removed
> 
> > From: Wu, Feng
> > Sent: Friday, November 18, 2016 12:27 PM
> > > > diff --git a/xen/drivers/passthrough/pci.c 
> > > > b/xen/drivers/passthrough/pci.c
> > > > index 8bce213..e71732f 100644
> > > > --- a/xen/drivers/passthrough/pci.c
> > > > +++ b/xen/drivers/passthrough/pci.c
> > > > @@ -1602,6 +1602,13 @@ int iommu_do_pci_domctl(
> > > >          break;
> > > >
> > > >      case XEN_DOMCTL_assign_device:
> > > > +        /* no domain_pause() */
> > > > +        if ( d == current->domain )
> > > > +        {
> > > > +            ret = -EINVAL;
> > > > +            break;
> > > > +        }
> > > > +
> > >
> > > don't understand why adding above check, and why "no domain_pause"
> > > matters in this change.
> >
> > In fact, this change is according Jan's following comments on v6:
> >
> > " There's one additional caveat here which no-one of us so far thought
> > of: Currently there's nothing preventing the domctl-s under which
> > this sits from being issued by the control domain for itself. Various
> > other domctl-s, however, guard against this case when intending
> > to pause the target domain. The same needs to be done for the
> > ones leading here."
> >
> > We need to prevent the domain from pausing itself.
> >
> 
> XEN_DOMCTL_assign_device doesn't imply a domain_pause operation,
> at least not obvious in this level. If we don't have PI enabled underneath,
> is above guard still necessary? If the answer is yes, the comment should
> be elaborated for easy understanding... :-)

I think the domain_pause() is introduced in PI related logic. So maybe I
need Jan's comments about whether we need to add this check unconditionally
here. Anyway, the comments need to be more detailed.

Thanks,
Feng

> 
> Thanks
> Kevin

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