On Fri, Dec 11 2020 at 13:10, Jürgen Groß wrote:

On 11.12.20 00:20, boris.ostrovsky@oracle.com wrote:

quoted

On 12/10/20 2:26 PM, Thomas Gleixner wrote:

quoted

All event channel setups bind the interrupt on CPU0 or the target CPU for
percpu interrupts and overwrite the affinity mask with the corresponding
cpumask. That does not make sense.

The XEN implementation of irqchip::irq_set_affinity() already picks a
single target CPU out of the affinity mask and the actual target is stored
in the effective CPU mask, so destroying the user chosen affinity mask
which might contain more than one CPU is wrong.

Change the implementation so that the channel is bound to CPU0 at the XEN
level and leave the affinity mask alone. At startup of the interrupt
affinity will be assigned out of the affinity mask and the XEN binding will
be updated.

If that's the case then I wonder whether we need this call at all and instead bind at startup time.

After some discussion with Thomas on IRC and xen-devel archaeology the
result is: this will be needed especially for systems running on a
single vcpu (e.g. small guests), as the .irq_set_affinity() callback
won't be called in this case when starting the irq.

That's right, but not limited to ARM. The same problem exists on x86 UP.
So yes, the call makes sense, but the changelog is not really useful.
Let me add a comment to this.

Thanks,

        tglx