question about pci_enable_msix_range

question about pci_enable_msix_range

[Julia Lawall]

Hello,

I was looking at the following code in the file
drivers/vfio/pci/vfio_pci_intrs.c:

                ret = pci_enable_msix_range(pdev, vdev->msix, 1, nvec);
                if (ret < nvec) {
                        if (ret > 0)
                                pci_disable_msix(pdev);
                        kfree(vdev->msix);
                        kfree(vdev->ctx);
                        return ret;
                }

I was wondering what is the point of using a range of 1 .. nvec if there
is going to be a failure if the number of allocated irqs is less than
nvec?

thanks,
julia

Re: question about pci_enable_msix_range

[Alex Williamson]

On Tue, 15 Mar 2016 11:26:50 +0100 (CET)
Julia Lawall <julia.lawall@lip6.fr> wrote:

> Hello,
> 
> I was looking at the following code in the file
> drivers/vfio/pci/vfio_pci_intrs.c:
> 
>                 ret = pci_enable_msix_range(pdev, vdev->msix, 1, nvec);
>                 if (ret < nvec) {
>                         if (ret > 0)
>                                 pci_disable_msix(pdev);
>                         kfree(vdev->msix);
>                         kfree(vdev->ctx);
>                         return ret;
>                 }
> 
> I was wondering what is the point of using a range of 1 .. nvec if there
> is going to be a failure if the number of allocated irqs is less than
> nvec?

Hi Julia,

The intention is that on failure we can indicate to the user a value
that might work.  If we were to call with {nvec, nvec} we'd only get
back -ENOSPC and the user could only arbitrarily decrease the request
by some amount and try again.  By using {1, nvec} we can hopefully
provide a useful next step.  On the other hand, we haven't enabled the
number of vectors the user requested, so it doesn't seem to make sense
to leave any enabled.  Thanks,

Alex

Re: question about pci_enable_msix_range

[Julia Lawall]

On Tue, 15 Mar 2016, Alex Williamson wrote:

> On Tue, 15 Mar 2016 11:26:50 +0100 (CET)
> Julia Lawall <julia.lawall@lip6.fr> wrote:
>
> > Hello,
> >
> > I was looking at the following code in the file
> > drivers/vfio/pci/vfio_pci_intrs.c:
> >
> >                 ret = pci_enable_msix_range(pdev, vdev->msix, 1, nvec);
> >                 if (ret < nvec) {
> >                         if (ret > 0)
> >                                 pci_disable_msix(pdev);
> >                         kfree(vdev->msix);
> >                         kfree(vdev->ctx);
> >                         return ret;
> >                 }
> >
> > I was wondering what is the point of using a range of 1 .. nvec if there
> > is going to be a failure if the number of allocated irqs is less than
> > nvec?
>
> Hi Julia,
>
> The intention is that on failure we can indicate to the user a value
> that might work.  If we were to call with {nvec, nvec} we'd only get
> back -ENOSPC and the user could only arbitrarily decrease the request
> by some amount and try again.  By using {1, nvec} we can hopefully
> provide a useful next step.  On the other hand, we haven't enabled the
> number of vectors the user requested, so it doesn't seem to make sense
> to leave any enabled.  Thanks,

Ah, OK I see now that the ret will be the value that works in this case,
not a normal failure value.  Thanks for the feedback, which has made
things a lot clearer.

julia