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

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Message-ID: <87k0go8432.ffs@tglx>
Date:   Wed, 01 Dec 2021 21:25:53 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Jiang <dave.jiang@...el.com>, 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 Wed, Dec 01 2021 at 11:47, Dave Jiang wrote:
> On 12/1/2021 11:41 AM, Thomas Gleixner wrote:
>>> 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.

Confused. Are you talking about passing a full IDXD device to the guest
or about passing a carved out subdevice, aka. queue?

Thanks,

        tglx