Re: [PATCH 2/2] nvme pci: try to allocate multiple irq vectors again in case of -EINVAL

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Dec 27, 2018 at 02:08:34PM +0100, Christoph Hellwig wrote:
> On Thu, Dec 27, 2018 at 04:21:38PM +0800, Ming Lei wrote:
> > On Wed, Dec 26, 2018 at 07:20:27PM +0100, Christoph Hellwig wrote:
> > > On Wed, Dec 26, 2018 at 06:37:55PM +0800, Ming Lei wrote:
> > > > It is observed on QEMU that pci_alloc_irq_vectors_affinity() may
> > > > returns -EINVAL when the requested number is too big(such as 64).
> > > 
> > > Which is not how this API is supposed to work and documented to work.
> > > 
> > > We need to fix pci_alloc_irq_vectors_affinity to not return a spurious
> > > error and just return the allocated number of vectors instead of
> > > hacking around that in drivers.
> > 
> > Yeah, you are right.
> > 
> > The issue is that QEMU nvme-pci is MSIX-capable only, and hasn't MSI
> > capability.
> > 
> > __pci_enable_msix_range() actually returns -ENOSPC, but __pci_enable_msi_range()
> > returns -EINVAL because dev->msi_cap is zero.
> > 
> > Maybe we need the following fix?
> 
> Should it matter?  We still get a negative vecs back, and still fall
> back to the next option.  

I'm not sure how it matters either, since
pci_alloc_irq_vectors_affinity() will fail either way.  It *would* be
nice to return the correct error in case the caller uses it to emit a
message.

But if the caller wants to use -ENOSPC to reduce @min_vecs and try
again, that sounds like an incorrect use of the interface -- the
caller should have just used the smaller @min_vecs the first time
around.

> Unless ther are no irqs available at all
> for the selected types pci_alloc_irq_vectors_affinity should never
> return an error.

I don't quite understand this last sentence.  If @min_vecs == 5 and
the device only supports 4 MSI-X and 4 MSI vectors, the function
comment says we should fail with -ENOSPC.

Bjorn