On Tue, Jul 8, 2014 at 6:26 AM, Alexander Gordeev [off-list ref] wrote:

On Mon, Jul 07, 2014 at 01:40:48PM -0600, Bjorn Helgaas wrote:

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Can you quantify the benefit of this?  Can't a device already use
MSI-X to request exactly the number of vectors it can use?  (I know

A Intel AHCI chipset requires 16 vectors written to MME while advertises
(via AHCI registers) and uses only 6. Even attempt to init 8 vectors results
in device's fallback to 1 (!).

Is the fact that it uses only 6 vectors documented in the public spec?

Yes, it is documented in ICH specs.

Out of curiosity, do you have a pointer to this?  It looks like it
uses one vector per port, and I'm wondering if the reason it requests
16 is because there's some possibility of a part with more than 8
ports.

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Is this a chipset erratum?  Are there newer versions of the chipset
that fix this, e.g., by requesting 8 vectors and using 6, or by also
supporting MSI-X?

No, this is not an erratum. The value of 8 vectors is reserved and could
cause undefined results if used.

As I read the spec (PCI 3.0, sec 6.8.1.3), if MMC contains 0b100
(requesting 16 vectors), the OS is allowed to allocate 1, 2, 4, 8, or
16 vectors.  If allocating 8 vectors and writing 0b011 to MME causes
undefined results, I'd say that's a chipset defect.

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I know this conserves vector numbers.  What does that mean in real
user-visible terms?  Are there systems that won't boot because of this
issue, and this patch fixes them?  Does it enable bigger
configurations, e.g., more I/O devices, than before?

Visibly, it ceases logging messages ('ahci 0000:00:1f.2: irq 107 for
MSI/MSI-X') for IRQs that are not shown in /proc/interrupts later.

No, it does not enable/fix any existing hardware issue I am aware of.
It just saves a couple of interrupt vectors, as Michael put it (10/16
to be precise). However, interrupt vectors space is pretty much scarce
resource on x86 and a risk of exhausting the vectors (and introducing
quota i.e) has already been raised AFAIR.

I'm not too concerned about the logging issue.  If necessary, we could
tweak that message somehow.

Interrupt vector space is the issue I would worry about, but I think
I'm going to put this on the back burner until it actually becomes a
problem.

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Do you know how Windows handles this?  Does it have a similar interface?

Have no clue, TBH. Can try to investigate if you see it helpful.

No, don't worry about investigating.  I was just curious because if
Windows *did* support something like this, that would be an indication
that there's a significant problem here and we might need to solve it,
too.  But it sounds like we can safely ignore it for now.

Bjorn