Messages in this thread | | | Date | Tue, 8 Jul 2014 14:26:07 +0200 | From | Alexander Gordeev <> | Subject | Re: [PATCH 1/3] PCI/MSI: Add pci_enable_msi_partial() |
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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.
> 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.
> 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.
> 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.
> As you can tell, I'm a little skeptical about this. It's a fairly big > change, it affects the arch interface, it seems to be targeted for > only a single chipset (though it's widely used), and we already > support a standard solution (MSI-X, reducing the number of vectors > requested, or even operating with 1 vector).
I also do not like the fact the arch interface is getting complicated, so I happily leave it to your judgement ;) Well, it is low-level and hidden from drivers at least.
Thanks!
> Bjorn
-- Regards, Alexander Gordeev agordeev@redhat.com
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