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

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Date:   Mon, 18 Sep 2023 11:17:05 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Shannon Nelson <shannon.nelson@....com>
Cc:     alex.williamson@...hat.com, kevin.tian@...el.com,
        reinette.chatre@...el.com, tglx@...utronix.de, 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 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.

If something is going wrong here it should be fixed in the
sriov_set_msix_vec_count() machinery, possibly in the pci core to
synchronize the msi_domain view of the world.

Jason