RE: [PATCH V4 10/11] vfio/pci: Support dynamic MSI-X

[Tian, Kevin]

> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Sent: Friday, May 5, 2023 11:28 PM
> 
> On Fri, 5 May 2023 08:10:33 +0000
> "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
> 
> > > From: Chatre, Reinette <reinette.chatre@xxxxxxxxx>
> > > Sent: Saturday, April 29, 2023 2:35 AM
> > >
> > > Hi Kevin,
> > >
> > > On 4/27/2023 11:50 PM, Tian, Kevin wrote:
> > >
> > > > Should this behavior be something configurable?
> > >
> > > This is not clear to me and I look to you for guidance here. From practical
> > > side it looks like configuration via module parameters is supported but
> > > whether it should be done is not clear to me.
> > >
> > > When considering this we need to think about what the user may expect
> > > when
> > > turning on/off the configuration. For example, MSI-X continues to
> allocate a
> > > range of interrupts during enabling. These have always been treated as a
> > > "cache" (interrupts remain allocated, whether they have an associated
> > > trigger
> > > or not). If there is new configurable behavior, do you expect that the
> > > driver needs to distinguish between the original "cache" that the user is
> > > used to and the new dynamic allocations? That is, should a dynamic MSI-
> X
> > > capable device always free interrupts when user space removes an
> eventfd
> > > or should only interrupts that were allocated dynamically be freed
> > > dynamically?
> >
> > That looks tricky. Probably that is why Alex suggested doing this simple
> > scheme and it is on par with the old logic anyway. So I'll withdraw this
> > comment.
> 
> Don't forget we're also releasing the irq reservations when the guest
> changes interrupt mode, ex. reboot, so the "caching" is really only
> within a session of the guest/userspace driver where it would be
> unusual to have an unused reservation for an extended period.  Thanks,
> 

Yeah, that makes sense.