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From:   "Tian, Kevin" <kevin.tian@intel.com>
To:     Thomas Gleixner <tglx@linutronix.de>,
        Jason Gunthorpe <jgg@nvidia.com>
CC:     LKML <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>, Joerg Roedel <joro@8bytes.org>,
        Will Deacon <will@kernel.org>,
        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        "Marc Zyngier" <maz@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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        "Williams, Dan J" <dan.j.williams@intel.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        "Raj, Ashok" <ashok.raj@intel.com>, Jon Mason <jdmason@kudzu.us>,
        Allen Hubbe <allenbh@gmail.com>
Subject: RE: [patch V2 31/33] iommu/vt-d: Enable PCI/IMS
Thread-Topic: [patch V2 31/33] iommu/vt-d: Enable PCI/IMS
Thread-Index: AQHY/bbwPG3EDS6e702OjDPxfqJk365NXjKQgAB3OYCAADxjgIAAAfkAgAWI4BA=
Date:   Mon, 28 Nov 2022 01:54:43 +0000
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> From: Thomas Gleixner <tglx@linutronix.de>
> Sent: Thursday, November 24, 2022 9:21 PM
>=20
> On Thu, Nov 24 2022 at 09:14, Jason Gunthorpe wrote:
> > On Thu, Nov 24, 2022 at 10:37:53AM +0100, Thomas Gleixner wrote:
> >> Jason said, that the envisioned Mellanox use case does not depend on t=
he
> >> IOMMU because the card itself has one which takes care of the
> >> protections.
> >
> > Right, but that doesn't mean we need the physical iommu turned
> > off. Setting the mlx pci device to identity mode is usually enough to
> > get back to full performance.
>=20
> Ok.

yes. IR can be enabled orthogonal to DMA mapping mode in IOMMU.

>=20
> >> How are we going to resolve that dilemma?
> >
> > The outcome is we don't have a strategy right now to make IMS work in
> > VMs. This series is all about making it work on physical machines,
> > that has to be a good first step.

yes, that is the point. As long as IMS is disabled in the guest it's alread=
y
a good first step for the moment.

> >
> > I'm hoping the OCP work stream on SIOV will tackle how to fix the
> > interrupt problems. Some of the ideas I've seen could be formed into
> > something that would work in a VM.
>=20
> Fair enough.
>=20
> Let me put the limitation into effect then.
>=20
> Thanks,
>=20
>         tglx