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SubjectRe: [PATCH 0/9] Support using MSI interrupts in ntb_transport

On 1/31/2019 3:39 PM, Logan Gunthorpe wrote:
> On 2019-01-31 1:58 p.m., Dave Jiang wrote:
>> On 1/31/2019 1:48 PM, Logan Gunthorpe wrote:
>>> On 2019-01-31 1:20 p.m., Dave Jiang wrote:
>>>> Does this work when the system moves the MSI vector either via software
>>>> (irqbalance) or BIOS APIC programming (some modes cause round robin
>>>> behavior)?
>>> I don't know how irqbalance works, and I'm not sure what you are
>>> referring to by BIOS APIC programming, however I would expect these
>>> things would not be a problem.
>>> The MSI code I'm presenting here doesn't do anything crazy with the
>>> interrupts, it allocates and uses them just as any PCI driver would. The
>>> only real difference here is that instead of a piece of hardware sending
>>> the IRQ TLP, it will be sent through the memory window (which, from the
>>> OS's perspective, is just coming from an NTB hardware proxy alias).
>>> Logan
>> Right. I did that as a hack a while back for some silicon errata
>> workaround. When the vector moves, the address for the LAPIC changes. So
>> unless it gets updated, you end up writing to the old location and lose
>> all the new interrupts. irqbalance is a user daemon that rotates the
>> system interrupts around to ensure that not all interrupts are pinned on
>> a single core.
> Yes, that would be a problem if something changes the MSI vectors out
> from under us. Seems like that would be a bit difficult to do even with
> regular hardware. So far I haven't seen anything that would do that. If
> you know of where in the kernel this happens I'd be interested in
> getting a pointer to the flow in the code. If that is the case this MSI
> stuff will need to get much more complicated...

I believe irqbalance writes to the file /proc/irq/N/smp_affinity. So
maybe take a look at the code that starts from there and see if it would
have any impact on your stuff.

>> I think it's enabled by default on several distros.
>> Although MSIX has nothing to do with the IOAPIC, the mode that the APIC
>> is programmed can have an influence on how the interrupts are delivered.
>> There are certain Intel platforms (I don't know if AMD does anything
>> like that) puts the IOAPIC in a certain configuration that causes the
>> interrupts to be moved in a round robin fashion. I think it's physical
>> flat mode? I don't quite recall. Normally on the low end Xeons. It's
>> probably worth doing a test run with the irqbalance daemon running and
>> make sure you traffic stream doesn't all of sudden stop.
> I've tested with irqbalance running and haven't found any noticeable
> difference.
> Logan

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