Re: [Xen-devel] [PATCH v3 4/6] Pause/Unpause the domain before/after assigning PI hooks

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

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Thursday, September 1, 2016 4:30 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
> Subject: Re: [PATCH v3 4/6] Pause/Unpause the domain before/after assigning
> PI hooks
> 
> >>> On 31.08.16 at 05:56, <feng.wu@xxxxxxxxx> wrote:
> > --- a/xen/arch/x86/hvm/vmx/vmx.c
> > +++ b/xen/arch/x86/hvm/vmx/vmx.c
> > @@ -219,8 +219,19 @@ void vmx_pi_hooks_assign(struct domain *d)
> >
> >      ASSERT(!d->arch.hvm_domain.vmx.vcpu_block);
> >
> > +    /*
> > +     * Pausing the domain can make sure the vCPU is not
> > +     * running and hence calling the hooks simultaneously
> > +     * when deassigning the PI hooks. This makes sure that
> > +     * all the appropriate state of PI descriptor is actually
> > +     * set up for all vCPus before leaving this function.
> > +     */
> > +    domain_pause(d);
> > +
> >      d->arch.hvm_domain.vmx.vcpu_block = vmx_vcpu_block;
> >      d->arch.hvm_domain.vmx.pi_do_resume = vmx_pi_do_resume;
> > +
> > +    domain_unpause(d);
> >  }
> 
> First of all I'm missing a word on whether the race mentioned in
> the description and comment can actually happen. Device
> (de)assignment should already be pretty much serialized (via
> the domctl lock, and maybe also via the pcidevs one).

The purpose of this patch is to address the race condition that
the _vCPU_ is running while we are installing these hooks. Do you
think this cannot happen?  This patch is trying to fix the issue
described at:
http://www.gossamer-threads.com/lists/xen/devel/433229
Consider that the other two hooks were installed when the VM
is created, seems no such race condition. However, according
to the discussion about patch 1 and patch 2 of series, we need
to install the other two hooks here as well, then the race
condition comes again, so we still need to handle it.

> 
> And then - isn't this overkill? Wouldn't a simple spin lock, taken
> here and in the deassign counterpart, do?
> 
> Or wait - is the comment perhaps wrongly talking about deassign?

Oh, yes, there are something wrong in the comments, this patch
has nothing to do with the _deassign_ stuff. The comments should
be like below:

+    /*
+     * Pausing the domain can make sure the vCPU is not
+     * running and hence calling the hooks simultaneously
+     * when _assigning_ the PI hooks. This makes sure that
+     * all the appropriate state of PI descriptor is actually
+     * set up for all vCPus before leaving this function.
+     */

Sorry for that.

> 
> If so the change is still questionable, as the hooks get set before
> the first device gets actually assigned to a guest (I remember
> that I insisted on things getting done that way when those
> original patches had been under review).

Yes, the hooks were installed before the first device gets assigned.
Then could you please elaborate what is the question here?

Thanks,
Feng

> 
> Jan


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