Re: [Xen-devel] Enabling VT-d PI by default

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On Fri, May 12, 2017 at 12:05 PM, Andrew Cooper
<andrew.cooper3@xxxxxxxxxx> wrote:
> Citrix Netscalar SDX boxes have more MSI-X interrupts than fit in the
> cumulative IDTs of a top end dual-socket Xeon server systems.  Some of
> the device drivers are purposefully modelled to use fewer interrupts
> than they otherwise would want to.
>
> Using PI is the proper solution longterm, because doing so would remove
> any need to allocate IDT vectors for the interrupts; the IOMMU could be
> programmed to dump device vectors straight into the PI block without
> them ever going through Xen's IDT.

I wouldn't necessarily call that a "proper" solution. With PI, instead
of an interrupt telling you exactly which VM to wake up and/or which
routine you need to run, instead you have to search through
(potentially) thousands of entries to see which vcpu the interrupt you
received wanted to wake up; and you need to do that on every single
interrupt.  (Obviously it does have the advantage that if the vcpu
happens to be running Xen doesn't get an interrupt at all.)

 -George

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