linux-kernel - Re: [PATCH v4 06/17] PCI: add SIOV and IMS capability detection

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Date:   Mon, 16 Nov 2020 21:37:27 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     "Tian\, Kevin" <kevin.tian@...el.com>,
        "Raj\, Ashok" <ashok.raj@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Wilk\, Konrad" <konrad.wilk@...cle.com>,
        "Williams\, Dan J" <dan.j.williams@...el.com>,
        "Jiang\, Dave" <dave.jiang@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        "vkoul\@kernel.org" <vkoul@...nel.org>,
        "Dey\, Megha" <megha.dey@...el.com>,
        "maz\@kernel.org" <maz@...nel.org>,
        "bhelgaas\@google.com" <bhelgaas@...gle.com>,
        "alex.williamson\@redhat.com" <alex.williamson@...hat.com>,
        "Pan\, Jacob jun" <jacob.jun.pan@...el.com>,
        "Liu\, Yi L" <yi.l.liu@...el.com>,
        "Lu\, Baolu" <baolu.lu@...el.com>,
        "Kumar\, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Luck\, Tony" <tony.luck@...el.com>,
        "kwankhede\@nvidia.com" <kwankhede@...dia.com>,
        "eric.auger\@redhat.com" <eric.auger@...hat.com>,
        "parav\@mellanox.com" <parav@...lanox.com>,
        "rafael\@kernel.org" <rafael@...nel.org>,
        "netanelg\@mellanox.com" <netanelg@...lanox.com>,
        "shahafs\@mellanox.com" <shahafs@...lanox.com>,
        "yan.y.zhao\@linux.intel.com" <yan.y.zhao@...ux.intel.com>,
        "pbonzini\@redhat.com" <pbonzini@...hat.com>,
        "Ortiz\, Samuel" <samuel.ortiz@...el.com>,
        "Hossain\, Mona" <mona.hossain@...el.com>,
        "dmaengine\@vger.kernel.org" <dmaengine@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci\@vger.kernel.org" <linux-pci@...r.kernel.org>,
        "kvm\@vger.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH v4 06/17] PCI: add SIOV and IMS capability detection

On Mon, Nov 16 2020 at 14:02, Jason Gunthorpe wrote:
> On Mon, Nov 16, 2020 at 06:56:33PM +0100, Thomas Gleixner wrote:
>> On Mon, Nov 16 2020 at 11:46, Jason Gunthorpe wrote:
>> 
>> > On Mon, Nov 16, 2020 at 07:31:49AM +0000, Tian, Kevin wrote:
>> >
>> >> > The subdevices require PASID & IOMMU in native, but inside the guest there
>> >> > is no
>> >> > need for IOMMU unless you want to build SVM on top. subdevices work
>> >> > without
>> >> > any vIOMMU or hypercall in the guest. Only because they look like normal
>> >> > PCI devices we could map interrupts to legacy MSIx.
>> >> 
>> >> Guest managed subdevices on PF/VF requires vIOMMU. 
>> >
>> > Why? I've never heard we need vIOMMU for our existing SRIOV flows in
>> > VMs??
>> 
>> Handing PF/VF into the guest does not require it.
>> 
>> But if the PF/VF driver in the guest wants to create and manage the
>> magic mdev subdevices which require PASID support then you surely need
>> it.
>
> 'magic mdevs' are only one reason to use IMS in a guest. On mlx5 we
> might want to use IMS for VPDA devices. mlx5 can spawn a VDPA device
> in a guest, against a 'ADI', without ever requiring an IOMMU to do it.
>
> We don't even need IOMMU in the hypervisor to create the ADI, mlx5 has
> an internal secure IOMMU that can be used instead of the platform
> IOMMU.
>
> Not saying this is a major use case, or a reason not to link things to
> IOMMU detection, but lets be clear that a hard need for IOMMU is a
> another IDXD thing, not general.

Fair enough.

Thanks,

        tglx