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

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Message-ID: <87a5tjxcva.ffs@tglx>
Date:   Tue, 19 Sep 2023 01:47:37 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jason Gunthorpe <jgg@...pe.ca>
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 20:37, Jason Gunthorpe wrote:
> 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.

Sorry. What I wanted to say is that the driver callback is not the right
place to reload the MSI domains after the change.

> 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?

Correct. So sriov_set_msix_vec_count() could just do:

	ret = pdev->driver->sriov_set_msix_vec_count(vf_dev, val);
        if (!ret)
        	teardown_msi_domain(pdev);

Right?

Thanks,

        tglx