linux-kernel - Re: question about pci_enable_msix

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Message-ID: <20160315082941.11216f19@t450s.home>
Date:	Tue, 15 Mar 2016 08:29:41 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Julia Lawall <julia.lawall@...6.fr>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alexander Gordeev <agordeev@...hat.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: question about pci_enable_msix_range

On Tue, 15 Mar 2016 11:26:50 +0100 (CET)
Julia Lawall <julia.lawall@...6.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