Re: [PATCH 2/2] Device assignment: Fix MSI IRQ affinity setting

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-05-24 18:39, Thomas Gleixner wrote:
> On Thu, 24 May 2012, Alex Williamson wrote:
>> On Thu, 2012-05-24 at 18:02 +0100, Richard Weinberger wrote:
>>> +            if (address == msi_start + PCI_MSI_DATA_32)
>>> +                handle_cfg_write_msi(pci_dev, assigned_dev);
>>
>> Why didn't we just use range_covers_byte(address, len, pci_dev->msi_cap
>> + PCI_MSI_DATA_32) to start with?  But how does this handle the enable
>> bit?
> 
> The problem with the current implementation is that it only changes
> the routing if the msi entry goes from masked to unmasked state.
> 
> Linux does not mask the entries on affinity changes and never did,
> neither for MSI nor for MSI-X.
> 
> I know it's probably not according to the spec, but we can't fix that
> retroactively.

For MSI, this is allowed. For MSI-X, this would clearly be a Linux bug,
waiting for hardware to dislike this spec violation.

However, if this is the current behavior of such a prominent guest, I
guess we have to stop optimizing the QEMU MSI-X code that it only
updates routings on mask changes. Possibly other OSes get this wrong too...

Thanks, for the clarification. Should go into the changelog.

Jan

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