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

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 06/12/2016 22:01, Stefano Stabellini wrote:
> On Tue, 6 Dec 2016, Stefano Stabellini wrote:
> > moving a vCPU with interrupts assigned to it is slower than moving a
> > vCPU without interrupts assigned to it. You could say that the
> > slowness is directly proportional do the number of interrupts assigned
> > to the vCPU.
> To be pedantic, by "assigned" I mean that a physical interrupt is routed
> to a given pCPU and is set to be forwarded to a guest vCPU running on it
> by the _IRQ_GUEST flag. The guest could be dom0. Upon receiving one of
> these physical interrupts, a corresponding virtual interrupt (could be a
> different irq) will be injected into the guest vCPU.
>
> When the vCPU is migrated to a new pCPU, the physical interrupts that
> are configured to be injected as virtual interrupts into the vCPU, are
> migrated with it. The physical interrupt migration has a cost. However,
> receiving physical interrupts on the wrong pCPU has an higher cost.
I don't understand why it is a problem for you to receive the first 
interrupt to the wrong pCPU and moving it if necessary.
While this may have an higher cost (I don't believe so) on the first 
received interrupt, migrating thousands of interrupts at the same time 
is very expensive and will likely get Xen stuck for a while (think about 
ITS with a single command queue).
Furthermore, the current approach will move every single interrupt 
routed a the vCPU, even those disabled. That's pointless and a waste of 
resource. You may argue that we can skip the ones disabled, but in that 
case what would be the benefits to migrate the IRQs while migrate the vCPUs?
So I would suggest to spread it over the time. This also means less 
headache for the scheduler developers.
Regards,

--
Julien Grall

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