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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Jason Gunthorpe <jgg@ziepe.ca>,
        Shannon Nelson <shannon.nelson@amd.com>
Cc:     alex.williamson@redhat.com, kevin.tian@intel.com,
        reinette.chatre@intel.com, kvm@vger.kernel.org,
        brett.creeley@amd.com, linux-kernel@vger.kernel.org,
        Leon Romanovsky <leonro@nvidia.com>
Subject: Re: [PATCH vfio] vfio/pci: remove msi domain on msi disable
In-Reply-To: <20230918141705.GE13795@ziepe.ca>
References: <20230914191406.54656-1-shannon.nelson@amd.com>
 <20230918141705.GE13795@ziepe.ca>
Date:   Mon, 18 Sep 2023 20:43:21 +0200
Message-ID: <87led3xqye.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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.

> 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.

Right, we should definitely not do that on a per driver basis.

Thanks,

        tglx