RE: Programming PASID in IMS entries

[Tian, Kevin]

> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Sent: Wednesday, July 7, 2021 4:51 PM
> 
> > Also, from a previous discussion [1], we want to make IMS more dynamic:
> >
> > Given the QEMU behavior it doesn't ask for all IRQs upfront. It only
> > allocates 1, and when it unmasks the 2nd, it wants to dynamically add a
> > second. This will allow adding a second IRQ without having to free all
> > the old irqs and reacquire the new number (as it is done today).
> >
> > This dynamic behavior is only for MSIx/IMS backed entries. For legacy
> > MSI, QEMU will allocate everything upfront. Since it has a
> > "num_vectors" enabled, nothing can be dynamically done for MSI. Kevin
> > is looking to have this fixed for legacy to stop the dynamic part for
> > MSI. We are pursuing this change just for IMS first, and once it
> > works, we can replicate the same for MSIx too.
> 
> No. Fix the existing stuff first and then IMS just works.
> 

Does below sound a plan?

1.  Fix Qemu to allocate all possible irqs when guest unmasks MSI/MSI-X,
    instead of freeing and re-allocating in the fly. This is an improvement
    for existing kernels which don't support dynamic resize.

2.  Extend MSI-X core to support dynamic resize. Via VFIO_IRQ_INFO_
    NORESIZE Qemu can enable dynamic resize if the flag is false. and
    from your comment dev-msi resize will be covered too  with this change.

3.  Add hypercall (or a pv irqchip) to provide feedback into guest in case 
    of irq shortage. This is also necessary to enable guest ims.

Thanks
Kevin