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

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Date:   Thu, 24 Nov 2022 14:21:04 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        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

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.

>> 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.
>
> 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