linux-kernel - Re: [patch 21/32] NTB/msi: Convert to msi_on_each

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Message-ID: <df00b87e-00dc-d998-8b64-46b16dba46eb@intel.com>
Date:   Wed, 1 Dec 2021 11:47:35 -0700
From:   Dave Jiang <dave.jiang@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Jason Gunthorpe <jgg@...dia.com>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Marc Zygnier <maz@...nel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Megha Dey <megha.dey@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-pci@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jon Mason <jdmason@...zu.us>, Allen Hubbe <allenbh@...il.com>,
        linux-ntb@...glegroups.com, linux-s390@...r.kernel.org,
        Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>, x86@...nel.org,
        Joerg Roedel <jroedel@...e.de>,
        iommu@...ts.linux-foundation.org
Subject: Re: [patch 21/32] NTB/msi: Convert to msi_on_each_desc()


On 12/1/2021 11:41 AM, Thomas Gleixner wrote:
> 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?

The hardware implementation does not have enough MSIX vectors for 
guests. There are only 9 MSIX vectors total (8 for queues) and 2048 IMS 
vectors. So if we are to do MSI-X for all of them, then we need to do 
the IMS backed MSIX scheme rather than passthrough IMS to guests.


>
> Thanks,
>
>          tglx
>