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

[Logan Gunthorpe]

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 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