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

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Message-ID: <BN9PR11MB52761AAC6808D9B8AC9C186D8C739@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Sat, 6 May 2023 08:15:00 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     "Chatre, Reinette" <reinette.chatre@...el.com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "yishaih@...dia.com" <yishaih@...dia.com>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "darwi@...utronix.de" <darwi@...utronix.de>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "tom.zanussi@...ux.intel.com" <tom.zanussi@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH V4 10/11] vfio/pci: Support dynamic MSI-X

> From: Alex Williamson <alex.williamson@...hat.com>
> Sent: Friday, May 5, 2023 11:28 PM
> 
> On Fri, 5 May 2023 08:10:33 +0000
> "Tian, Kevin" <kevin.tian@...el.com> wrote:
> 
> > > From: Chatre, Reinette <reinette.chatre@...el.com>
> > > 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.