linux-kernel - Re: [PATCH vfio] vfio/pci: remove msi domain on msi disable

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Message-ID: <20230918233735.GP13795@ziepe.ca>
Date:   Mon, 18 Sep 2023 20:37:35 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Shannon Nelson <shannon.nelson@....com>,
        alex.williamson@...hat.com, kevin.tian@...el.com,
        reinette.chatre@...el.com, kvm@...r.kernel.org,
        brett.creeley@....com, linux-kernel@...r.kernel.org,
        Leon Romanovsky <leonro@...dia.com>
Subject: Re: [PATCH vfio] vfio/pci: remove msi domain on msi disable

On Mon, Sep 18, 2023 at 08:43:21PM +0200, Thomas Gleixner wrote:
> On Mon, Sep 18 2023 at 11:17, Jason Gunthorpe wrote:
> > On Thu, Sep 14, 2023 at 12:14:06PM -0700, Shannon Nelson wrote:
> >> The new MSI dynamic allocation machinery is great for making the irq
> >> management more flexible.  It includes caching information about the
> >> MSI domain which gets reused on each new open of a VFIO fd.  However,
> >> this causes an issue when the underlying hardware has flexible MSI-x
> >> configurations, as a changed configuration doesn't get seen between
> >> new opens, and is only refreshed between PCI unbind/bind cycles.
> >> 
> >> In our device we can change the per-VF MSI-x resource allocation
> >> without the need for rebooting or function reset.  For example,
> >> 
> >>   1. Initial power up and kernel boot:
> >> 	# lspci -s 2e:00.1 -vv | grep MSI-X
> >> 	        Capabilities: [a0] MSI-X: Enable+ Count=8 Masked-
> >> 
> >>   2. Device VF configuration change happens with no reset
> >
> > Is this an out of tree driver problem?
> >
> > The intree way to alter the MSI configuration is via
> > sriov_set_msix_vec_count, and there is only one in-tree driver that
> > uses it right now.
> 
> Right, but that only addresses the driver specific issues.

Sort of.. sriov_vf_msix_count_store() is intended to be the entry
point for this and if the kernel grows places that cache the value or
something then this function should flush those caches too.

I suppose flushing happens implicitly because Shannon reports that
things work fine if the driver is rebound. Since
sriov_vf_msix_count_store() ensures there is no driver bound before
proceeding it probe/unprobe must be flushing out everything?

Jason