Re: [PATCH v2] PCI/MSI: Re-add checks for skip masking MSI-X on Xen PV

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

On Mon, 2021-10-25 at 21:21 +0200, Josef Johansson wrote:
> +       if (!(pci_msi_ignore_mask || entry->msi_attrib.is_virtual))

Is it just me, or is that a lot easier to read if you write it as the
tautologically-identical (!pci_msi_ignore_mask && !entry->…is_virtual)?


> @@ -546,7 +548,8 @@ static int msi_capability_init(struct pci_dev *dev, int 
> nvec,
>                 return -ENOMEM;

>         /* All MSIs are unmasked by default; mask them all *
> -       pci_msi_mask(entry, msi_multi_mask(entry))
> +       if (!pci_msi_ignore_mask)
> +               pci_msi_mask(entry, msi_multi_mask(entry));
>
>         list_add_tail(&entry->list, dev_to_msi_list(&dev->dev));


Hm, I thought that older kernels *did* do this part (or at least the
later ones in pci_msi*_hutdown). I was watching it when I did the Xen
hosting implementation I mentioned before; even a hack to unmask them
all when the VM was started, wasn't working because the guest would
*mask* all MSI-X, just never unmask them again.


I wonder if we should rename 'pci_msi_ignore_mask' to something with
Xen in its name because Xen is the only user of this abomination (which
fundamentally seems to require that the virtual hardware use MSI
entries even while they're masked, so hopefully nobody else would
*ever* do such a thing), and the required behaviour is very Xen-
specific.

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