Re: [PATCH RFC v2 02/18] irq/dev-msi: Add support for a new DEV_MSI irq domain

[Jason Gunthorpe]

On Thu, Aug 06, 2020 at 10:21:11PM +0200, Thomas Gleixner wrote:

> Optionally? Please tell the hardware folks to make this mandatory. We
> have enough pain with non maskable MSI interrupts already so introducing
> yet another non maskable interrupt trainwreck is not an option.

Can you elaborate on the flows where Linux will need to trigger
masking?

I expect that masking will be available in our NIC HW too - but it
will require a spin loop if masking has to be done in an atomic
context.

> It's more than a decade now that I tell HW people not to repeat the
> non-maskable MSI failure, but obviously they still think that
> non-maskable interrupts are a brilliant idea. I know that HW folks
> believe that everything they omit can be fixed in software, but they
> have to finally understand that this particular issue _cannot_ be fixed
> at all.

Sure, the CPU should always be able to shut off an interrupt!

Maybe explaining the goals would help understand the HW perspective.

Today HW can process > 100k queues of work at once. Interrupt delivery
works by having a MSI index in each queue's metadata and the interrupt
indirects through a MSI-X table on-chip which has the
addr/data/mask/etc.

What IMS proposes is that the interrupt data can move into the queue
meta data (which is not required to be on-chip), eg along side the
producer/consumer pointers, and the central MSI-X table is not
needed. This is necessary because the PCI spec has very harsh design
requirements for a MSI-X table that make scaling it prohibitive.

So an IRQ can be silenced by deleting or stopping the queue(s)
triggering it. It can be masked by including masking in the queue
metadata. We can detect pending by checking the producer/consumer
values.

However synchronizing all the HW and all the state is now more
complicated than just writing a mask bit via MMIO to an on-die memory.

Jason