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Re: [Xen-devel] [RFC PATCH 21/24] ARM: vITS: handle INVALL command



On Tue, 6 Dec 2016, Julien Grall wrote:
> > > This window may be bigger with LPIs, because a single vCPU may have
> > > thousand
> > > interrupts routed. This would take a long time to move all of them when
> > > the
> > > vCPU is migrating. So we may want to take a lazy approach and moving them
> > > when
> > > they are received on the "wrong" pCPU.
> > 
> > That's possible. The only downside is that modifying the irq migration
> > workflow is difficult and we might want to avoid it if possible.
> 
> I don't think this would modify the irq migration work flow. If you look at
> the implementation of arch_move_irqs, it will just go over the vIRQ and call
> irq_set_affinity.
> 
> irq_set_affinity will directly modify the hardware and that's all.
> 
> > 
> > Another approach is to let the scheduler know that migration is slower.
> > In fact this is not a new problem: it can be slow to migrate interrupts,
> > even few non-LPIs interrupts, even on x86. I wonder if the Xen scheduler
> > has any knowledge of that (CC'ing George and Dario). I guess that's the
> > reason why most people run with dom0_vcpus_pin.
> 
> I gave a quick look at x86, arch_move_irqs is not implemented. Only PIRQ are
> migrated when a vCPU is moving to another pCPU.
> 
> The function pirq_set_affinity, will change the affinity of a PIRQ but only in
> software (see irq_set_affinity). This is not yet replicated the configuration
> into the hardware.
> 
> In the case of ARM, we directly modify the configuration of the hardware. This
> adds much more overhead because you have to do an hardware access for every
> single IRQ.

George, Dario, any comments on whether this would make sense and how to
do it?

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