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

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Message-ID: <878rx480fk.ffs@tglx>
Date:   Wed, 01 Dec 2021 22:44:47 +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 14:21, Dave Jiang wrote:
> On 12/1/2021 1:25 PM, Thomas Gleixner wrote:
>>> 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?
>
> I'm talking about carving out a subdevice. I had the impression of you 
> wanting IMS passed through for all variations. But it sounds like for a 
> sub-device, you are ok with the implementation of MSIX backed by IMS?

I don't see anything wrong with that. A subdevice is it's own entity and
VFIO can chose the most conveniant representation of it to the guest
obviously.

How that is backed on the host does not really matter. You can expose
MSI-X to the guest with a INTx backing as well.

I'm still failing to see the connection between the 9 MSIX vectors and
the 2048 IMS vectors which I assume that this is the limitation of the
physical device, right?

What needs a subdevice to expose?

Thanks,

        tglx