linux-kernel - RE: [patch V2 31/33] iommu/vt-d: Enable PCI/IMS

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Date:   Mon, 28 Nov 2022 01:54:43 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Jason Gunthorpe <jgg@...dia.com>
CC:     LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Will Deacon <will@...nel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        "Marc Zyngier" <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        "Raj, Ashok" <ashok.raj@...el.com>, Jon Mason <jdmason@...zu.us>,
        Allen Hubbe <allenbh@...il.com>
Subject: RE: [patch V2 31/33] iommu/vt-d: Enable PCI/IMS

> From: Thomas Gleixner <tglx@...utronix.de>
> Sent: Thursday, November 24, 2022 9:21 PM
> 
> 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 the
> >> 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.
> 
> Ok.

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

> 
> >> 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 already
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.
> 
> Fair enough.
> 
> Let me put the limitation into effect then.
> 
> Thanks,
> 
>         tglx