Re: [Xen-devel] [PATCH v2 3/4] VMX: Assign the right value to 'NDST' field in a concern case

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

> -----Original Message-----
> From: dunlapg@xxxxxxxxx [mailto:dunlapg@xxxxxxxxx] On Behalf Of George
> Dunlap
> Sent: Thursday, June 23, 2016 2:01 AM
> To: Wu, Feng <feng.wu@xxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxx; Tian, Kevin <kevin.tian@xxxxxxxxx>; Keir Fraser
> <keir@xxxxxxx>; Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; Dario Faggioli
> <dario.faggioli@xxxxxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>
> Subject: Re: [Xen-devel] [PATCH v2 3/4] VMX: Assign the right value to 'NDST'
> field in a concern case
> 
> On Thu, May 26, 2016 at 2:39 PM, Feng Wu <feng.wu@xxxxxxxxx> wrote:
> > Normally, in vmx_cpu_block() 'NDST' filed should have the same
> > value with 'dest' or 'MASK_INSR(dest, PI_xAPIC_NDST_MASK)' depending
> > on whether x2apic is enabled. However, in the following scenario,
> > 'NDST' has different value:
> >
> > 'vcpu_block' hook gets assigned in vmx_pi_hooks_assign(), but all
> > other three PI hooks have not been assigned or not been excuted yet.
> > And during this interval, we are running in vmx_vcpu_block(), then
> > 'NDST' may have different value.
> >
> > This patch fix this concern case.
> >
> > Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>
> 
> I agree with Jan that a cleaner solution here would be making sure
> that all the appropriate state is actually set up for all vcpus before
> leaving vmx_pi_hooks_assign().  With the patch you propose, the
> following sequence of events is possible:
> 
> * vcpu 0 starts running on a pcpu
> * a device is assigned, causing the hooks to be set
> * an interrupt from the device is routed to vcpu 0, but it is not
> actually delivered properly, since ndst is not pointing to the right
> processor.
> 
> One option would be to pause all vcpus before setting the hooks and
> then un-pause them; this would force all the vcpus to go through
> vmx_pi_switch_to() before vmx_vcpu_block().  Another would be to grab
> the scheduler lock for each pcpu and write the vcpu's ndst with the
> appropriate value before setting the hooks.

That sounds a great idea. Besides that, maybe we can also pause/unpause
the domain before/after unsetting the hooks, then we don't need to
care about the race condition when vmx_pi_hooks_deassign() and
vmx_vcpu_block() get called at the same time. After unpause the domain,
we can safely remove the vCPUs from the per-cpu blocking list if needed.

Thanks,
Feng

> 
>  -George
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