Re: [Xen-devel] [PATCH 1/3] PCI/MSI: Add pci_enable_msi_partial()

[Bjorn Helgaas <bhelgaas@xxxxxxxxxx>]

On Tue, Jul 8, 2014 at 6:26 AM, Alexander Gordeev <agordeev@xxxxxxxxxx> wrote:
> On Mon, Jul 07, 2014 at 01:40:48PM -0600, Bjorn Helgaas wrote:
>> >> 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.

>> 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.

>> 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.

>> 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

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