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

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Message-ID: <BN9PR11MB52765F2EF8420C60FD5945D18C709@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Thu, 9 Dec 2021 05:23:42 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        "Jiang, Dave" <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>,
        "Dey, Megha" <megha.dey@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "linux-pci@...r.kernel.org" <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-ntb@...glegroups.com>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        "Christian Borntraeger" <borntraeger@...ibm.com>,
        "x86@...nel.org" <x86@...nel.org>, Joerg Roedel <jroedel@...e.de>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>
Subject: RE: [patch 21/32] NTB/msi: Convert to msi_on_each_desc()

> From: Thomas Gleixner <tglx@...utronix.de>
> Sent: Thursday, December 2, 2021 5:45 AM
> 
> 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.
> 

Agree with this point. How the interrupts are represented to the guest
is orthogonal to how the backend resource is allocated. Physically MSI-X 
and IMS can be enabled simultaneously on an IDXD device. Once 
dynamic allocation is allowed for both, either one can be allocated for
a subdevice (with only difference on supported #subdevices). 

When an interrupt resource is exposed to the guest with the same type 
(e.g. MSI-on-MSI or IMS-on-IMS), it can be also passed through to the 
guest as long as a hypercall machinery is in place to get addr/data pair 
from the host (as you suggested earlier).

Thanks
Kevin