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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     "Tian, Kevin" <kevin.tian@intel.com>,
        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>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        "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
In-Reply-To: <Y39uGVLrH3MvOoZ4@nvidia.com>
References: <20221121083657.157152924@linutronix.de>
 <20221121091328.184455059@linutronix.de>
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 <87ilj4d766.ffs@tglx> <Y39uGVLrH3MvOoZ4@nvidia.com>
Date:   Thu, 24 Nov 2022 14:21:04 +0100
Message-ID: <878rk0cwu7.ffs@tglx>
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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