RE: Virtualizing MSI-X on IMS via VFIO

[Thomas Gleixner]

Kevin,

On Thu, Jun 24 2021 at 00:00, Kevin Tian wrote:
>> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
>> Sent: Wednesday, June 23, 2021 11:20 PM
>>
> [...]
>  > > So the only downside today of allocating more MSI-X vectors than
>> > necessary is memory consumption for the irq descriptors.
>> 
>> As above, this is a QEMU policy of essentially trying to be a good
>> citizen and allocate only what we can infer the guest is using.  What's
>> a good way for QEMU, or any userspace, to know it's running on a host
>> where vector exhaustion is not an issue?
>
> In my proposal a new command (VFIO_DEVICE_ALLOC_IRQS) is
> introduced to separate allocation from enabling. The availability
> of this command could be the indicator whether vector 
> exhaustion is not an issue now?

Your proposal still does not address the fundamental issue of a missing
feedback to the guest and you can invent a gazillion more IOCTL commands
and none of them will solve that issue. A hypercall/paravirt interface is
the only reasonable solution.

The time you are wasting to come up with non-solutions would have surely
been better spent implementing the already known and obvious proper
solution. You might be halfways done already with that.

Thanks,

        tglx