Re: [PATCH 0/4 v9] MSI-X MMIO support for KVM

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

On 2011-09-06 11:00, Sasha Levin wrote:
> On Tue, 2011-09-06 at 10:49 +0200, Jan Kiszka wrote:
>> On 2011-09-06 10:46, Sasha Levin wrote:
>>> On Tue, 2011-09-06 at 10:36 +0200, Jan Kiszka wrote:
>>>> On 2011-09-06 10:12, Michael S. Tsirkin wrote:
>>>>> On Tue, Sep 06, 2011 at 10:52:41AM +0300, Avi Kivity wrote:
>>>>>>>
>>>>>>> BTW, the same is also true for that optional per-vector masking of
>>>>>>> legacy MSI. Are there devices in the field that actually support this? I
>>>>>>> haven't found one so far and tend to consider this feature not worth
>>>>>>> implementing.
>>>>>>
>>>>>> Don't know.  I don't like implementing features on the basis of bug
>>>>>> reports, though.  On the other hand we can't really test it without
>>>>>> a real device.
>>>>>
>>>>> Linux will use this capability if present. So
>>>>> we could add support for an emulated device (e.g. e1000),
>>>>> then test with nested virt once iommu emulation lands :)
>>>>
>>>> Yeah, would be kind of cool. Still, I would feel better having it tested
>>>> against a real silicon as well. Also to prove that there is a real need.
>>>>
>>>> So, in case someone stumbles for such a device (bit 8 set in MSI control
>>>> word), please let us know!
>>>
>>> Doesn't any device that supports MSI-X supports per-vector masking?
>>>
>>> >From the spec:
>>>
>>> "MSI and MSI-X each support per-vector masking. Per-vector masking is an
>>> optional extension to MSI, and a standard feature with MSI-X"
>>
>> Right, but the cap flag still has to be set.
>>
> 
> Are you sure? Take a look at the table in section 6.8.2.3 in the spec
> (pci v3). Unlike the message control for MSI, this table doesn't mention
> anything about bit 8 or the per-vector masking capability for MSI-X, it
> just assumes it's there.

[Err, I should stop doing n things in parallel.]

Of course, MSI-X implies per-vector masking, but in a totally different
way with different data structures etc. That's not interesting for the
case in question: per-vector masking for legacy MSI.

Back to square #1: We need a device with MSI support and cap bit 8 set
in its _MSI_ control word.

Jan

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