RE: Question: interrupt randomness and handle_percpu_devid_irq()

[Michael Kelley]

From: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Sent: Wednesday, April 1, 2026 7:14 AM
> On 2026-03-20 16:10:37 [+0100], Thomas Gleixner wrote:
> > On Thu, Mar 19 2026 at 19:34, Michael Kelley wrote:
> > > The function header comment for handle_percpu_devid_irq() says that it is the
> > > same as handle_percpu_irq(), but with the addition of a pointer to a percpu
> > > variable with the real device id. That makes sense. But there's another difference:
> > > handle_percpu_irq() calls add_interrupt_randomness() [via handle_irq_event_percpu()],
> > > while handle_percpu_devid_irq() does not.
> > >
> > > Question: Is there a reason for this difference in handling interrupt randomness?
> > > Or is it just an oversight? handle_percpu_devid_irq() is used, for example, for the
> > > SGIs and PPIs on the GICv3 chip, so I wondered if IPIs (as built on SGIs) & PPIs
> > > specifically did not want the overhead of add_interrupt_randomness(). But then
> > > GICv5 is doing IPIs using LPIs, which use handle_percpu_irq() and hence *do*
> > > add interrupt randomness. That seemed inconsistent, which didn't help provide
> > > an answer.
> > >
> > > The question arises in the context of Linux guests running on Hyper-V. Hyper-V
> > > VMBus interrupts to the guest are per-CPU interrupts in Linux, using a PPI on
> > > arm64. So these interrupts do not call add_interrupt_randomness(), which is a
> > > problem because these guests don't have much other way to get entropy. To
> > > fix this, the VMBus ISR has always had an explicit call to
> > > add_interrupt_randomness(). But maybe that's not the best approach, and
> > > handle_percpu_devid_irq() should be fixed to call add_interrupt_randomness().
> >
> > I don't think there is a real good reason unless any of those interrupts
> > is NMI like.
> 
> So we could wire up the generic/ arch code here and feed the
> "randomness" from there and it from the driver bits.
> 

I'm planning to submit patches to do that, and remove the
add_interrupt_randomness() calls from the VMBus ISRs.  Hopefully will
get the patches out today or tomorrow.

Michael