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

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Message-ID: <MWHPR11MB1645E87DBEFFCC017C4849CC8CE30@MWHPR11MB1645.namprd11.prod.outlook.com>
Date:   Mon, 16 Nov 2020 23:51:35 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Thomas Gleixner <tglx@...utronix.de>
CC:     "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@...nel.org" <vkoul@...nel.org>,
        "Dey, Megha" <megha.dey@...el.com>,
        "maz@...nel.org" <maz@...nel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "alex.williamson@...hat.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@...dia.com" <kwankhede@...dia.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
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        "Ortiz, Samuel" <samuel.ortiz@...el.com>,
        "Hossain, Mona" <mona.hossain@...el.com>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
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        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: RE: [PATCH v4 06/17] PCI: add SIOV and IMS capability detection

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Tuesday, November 17, 2020 2:03 AM
> 
> 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.
> 

I should use "may require" in original post. and one thing that I obviously
mixed is the requirement of PASID-granular interrupt isolation in the
physical IOMMU instead of virtual IOMMU. But anyway, I didn't attempt
to use above to build hard need for IOMMU, just the opposite when looking
at all three cases together.

btw Jason/Thomas, how do you think about the proposal down in this
thread (ims=[auto|on|off])? Does it sound a good tradeoff to move forward?

Thanks
Kevin