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

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

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.

 -George

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