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From: Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [patch 21/32] NTB/msi: Convert to msi_on_each_desc()
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Date: Wed, 01 Dec 2021 19:41:05 +0100
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To: Dave Jiang <dave.jiang@intel.com>, Jason Gunthorpe <jgg@nvidia.com>
Cc: Logan Gunthorpe <logang@deltatee.com>, LKML <linux-kernel@vger.kernel.org>, Bjorn Helgaas <helgaas@kernel.org>, Marc Zygnier <maz@kernel.org>, Alex Williamson <alex.williamson@redhat.com>, Kevin Tian <kevin.tian@intel.com>, Megha Dey <megha.dey@intel.com>, Ashok Raj <ashok.raj@intel.com>, linux-pci@vger.kernel.org, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Jon Mason <jdmason@kudzu.us>, Allen Hubbe <allenbh@gmail.com>, linux-ntb@googlegroups.com, linux-s390@vger.kernel.org, Heiko Carstens <hca@linux.ibm.com>, Christian Borntraeger <borntraeger@de.ibm.com>, x86@kernel.org, Joerg Roedel <jroedel@suse.de>, iommu@lists.linux-foundation.org
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Dave,

please trim your replies.

On Wed, Dec 01 2021 at 09:28, Dave Jiang wrote:

> On 12/1/2021 3:16 AM, Thomas Gleixner wrote:
>> Jason,
>>
>> CC+ IOMMU folks
>>
>> On Tue, Nov 30 2021 at 20:17, Jason Gunthorpe wrote:
>>> On Tue, Nov 30, 2021 at 10:23:16PM +0100, Thomas Gleixner wrote:
>>
>> Though I fear there is also a use case for MSI-X and IMS tied to the
>> same device. That network card you are talking about might end up using
>> MSI-X for a control block and then IMS for the actual network queues
>> when it is used as physical function device as a whole, but that's
>> conceptually a different case.
>
> Hi Thomas. This is actually the IDXD usage for a mediated device passed 
> to a guest kernel when we plumb the pass through of IMS to the guest 
> rather than doing previous implementation of having a MSIX vector on 
> guest backed by IMS.

Which makes a lot of sense.

> The control block for the mediated device is emulated and therefore an
> emulated MSIX vector will be surfaced as vector 0. However the queues
> will backed by IMS vectors. So we end up needing MSIX and IMS coexist
> running on the guest kernel for the same device.

Why? What's wrong with using straight MSI-X for all of them?

Thanks,

        tglx