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
