INTMS/INTMC not being used in NVME interrupt handling

INTMS/INTMC not being used in NVME interrupt handling

[Bharat Kumar Gogada]

Hi,

As per NVME specification: 
7.5.1.1 Host Software Interrupt Handling
It is recommended that host software utilize the Interrupt Mask Set and Interrupt Mask Clear (INTMS/INTMC) 
registers to efficiently handle interrupts when configured to use pin based or MSI messages.

In kernel 4.14, drivers/nvme/host/pci.c function nvme_isr 
doesn't  use these registers. 

Any reason why these registers are not used in nvme interrupt handler ?

Why NVMe driver is not using any bottom half and processing all completion queues
in interrupt handler ?

Regards,
Bharat

Re: INTMS/INTMC not being used in NVME interrupt handling

[Keith Busch]

On Wed, May 16, 2018 at 12:35:15PM +0000, Bharat Kumar Gogada wrote:
> Hi,
> 
> As per NVME specification: 
> 7.5.1.1 Host Software Interrupt Handling
> It is recommended that host software utilize the Interrupt Mask Set and Interrupt Mask Clear (INTMS/INTMC) 
> registers to efficiently handle interrupts when configured to use pin based or MSI messages.
> 
> In kernel 4.14, drivers/nvme/host/pci.c function nvme_isr 
> doesn't  use these registers. 
> 
> Any reason why these registers are not used in nvme interrupt handler ?

I think you've answered your own question: we process completions in the
interrupt context. The interrupt is already masked at the CPU level in
this context, so there should be no reason to mask them at the device
level.
 
> Why NVMe driver is not using any bottom half and processing all completion queues
> in interrupt handler ?

Performance.

RE: INTMS/INTMC not being used in NVME interrupt handling

[Bharat Kumar Gogada]

> > Hi,
> >
> > As per NVME specification:
> > 7.5.1.1 Host Software Interrupt Handling It is recommended that host
> > software utilize the Interrupt Mask Set and Interrupt Mask Clear
> > (INTMS/INTMC) registers to efficiently handle interrupts when configured
> to use pin based or MSI messages.
> >
> > In kernel 4.14, drivers/nvme/host/pci.c function nvme_isr doesn't  use
> > these registers.
> >
> > Any reason why these registers are not used in nvme interrupt handler ?
> 
> I think you've answered your own question: we process completions in the
> interrupt context. The interrupt is already masked at the CPU level in this
> context, so there should be no reason to mask them at the device level.
> 
> > Why NVMe driver is not using any bottom half and processing all
> > completion queues in interrupt handler ?
> 
> Performance.
Thanks keith. 
Currently driver isn't setting any Coalesce count.  
So the NVMe card will raise interrupt for every single completion queue ?

For legacy interrupt for each CQ 
CQ-> ASSERT_INTA-> DOORBELL-> DEASSERT_INTA is this flow correct ?

Is the following flow valid
CQ1->ASSERT_INTA->CQ2/CQ3->Doorbell->DEASSERT_INTA ?

When using legacy interrupts, if CQ1 is sent followed by ASSERT_INTA, can the EP send 
another CQ2,CQ3.. before DEASSERT_INTA of CQ1 is generated?

Regards,
Bharat

Re: INTMS/INTMC not being used in NVME interrupt handling

[Keith Busch]

On Thu, May 17, 2018 at 11:15:59AM +0000, Bharat Kumar Gogada wrote:
> > > Hi,
> > >
> > > As per NVME specification:
> > > 7.5.1.1 Host Software Interrupt Handling It is recommended that host
> > > software utilize the Interrupt Mask Set and Interrupt Mask Clear
> > > (INTMS/INTMC) registers to efficiently handle interrupts when configured
> > to use pin based or MSI messages.
> > >
> > > In kernel 4.14, drivers/nvme/host/pci.c function nvme_isr doesn't  use
> > > these registers.
> > >
> > > Any reason why these registers are not used in nvme interrupt handler ?
> > 
> > I think you've answered your own question: we process completions in the
> > interrupt context. The interrupt is already masked at the CPU level in this
> > context, so there should be no reason to mask them at the device level.
> > 
> > > Why NVMe driver is not using any bottom half and processing all
> > > completion queues in interrupt handler ?
> > 
> > Performance.
> Thanks keith. 
> Currently driver isn't setting any Coalesce count.  
> So the NVMe card will raise interrupt for every single completion queue ?
> 
> For legacy interrupt for each CQ 
> CQ-> ASSERT_INTA-> DOORBELL-> DEASSERT_INTA is this flow correct ?

Mostly, yes. There could be a case where the controller wouldn't
deassert INTx if there are more completes past the CQ head doorbell write.

> Is the following flow valid
> CQ1->ASSERT_INTA->CQ2/CQ3->Doorbell->DEASSERT_INTA ?
> 
> When using legacy interrupts, if CQ1 is sent followed by ASSERT_INTA, can the EP send 
> another CQ2,CQ3.. before DEASSERT_INTA of CQ1 is generated?

I assume you are saying CQ entry 1, CQ entry 2, etc ...

The end point may continue posting those completion queue entries while
the interrupt is asserted. It should not deassert the interrupt until
the host acknowledges all outstanding completions with a CQ doorbell
write.