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

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

On 27 October 2021 09:13:36 BST, Josef Johansson <josef@xxxxxxxxxxx> wrote:
>On 10/27/21 08:24, David Woodhouse wrote:
>> 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)?
>Sure, the less parentheses the better.
>>
>>> @@ -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.
>When you're saying *did* here, do you mean that they mask even though
>pci_msi_ignore_mask = 0?
>
>As I was looking through pre Thomas' changes and post, it seems that we
>did indeed
>check for pci_msi_ignore_mask in msi_capability_init.


Ah, maybe not so old. When my VMM part didn't work with standard ancient distro 
test images I turned to a relatively current git HEAD.

So I was probably just unfortunate, and masking MSI under Xen at setup time was 
a temporary aberration; on older kernels the hack of enabling each vector at 
startup might have worked?

I'll disable my eventual VMM-side fix and retest different guest kernels to be 
sure (and to make sure I have the full permutation set for regression testing 
because regardless of how insane Xen's behaviour is, I need to faithfully 
emulate it for every Linux kernel behaviour that existed).


-- 
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