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

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Date:   Mon, 2 Nov 2020 09:21:58 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Dave Jiang <dave.jiang@...el.com>
CC:     Bjorn Helgaas <helgaas@...nel.org>, <vkoul@...nel.org>,
        <megha.dey@...el.com>, <maz@...nel.org>, <bhelgaas@...gle.com>,
        <tglx@...utronix.de>, <alex.williamson@...hat.com>,
        <jacob.jun.pan@...el.com>, <ashok.raj@...el.com>,
        <yi.l.liu@...el.com>, <baolu.lu@...el.com>, <kevin.tian@...el.com>,
        <sanjay.k.kumar@...el.com>, <tony.luck@...el.com>,
        <jing.lin@...el.com>, <dan.j.williams@...el.com>,
        <kwankhede@...dia.com>, <eric.auger@...hat.com>,
        <parav@...lanox.com>, <rafael@...nel.org>, <netanelg@...lanox.com>,
        <shahafs@...lanox.com>, <yan.y.zhao@...ux.intel.com>,
        <pbonzini@...hat.com>, <samuel.ortiz@...el.com>,
        <mona.hossain@...el.com>, <dmaengine@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-pci@...r.kernel.org>,
        <kvm@...r.kernel.org>
Subject: Re: [PATCH v4 06/17] PCI: add SIOV and IMS capability detection

On Fri, Oct 30, 2020 at 03:49:22PM -0700, Dave Jiang wrote:
> 
> 
> On 10/30/2020 3:45 PM, Jason Gunthorpe wrote:
> > On Fri, Oct 30, 2020 at 02:20:03PM -0700, Dave Jiang wrote:
> > > So the intel-iommu driver checks for the SIOV cap. And the idxd driver
> > > checks for SIOV and IMS cap. There will be other upcoming drivers that will
> > > check for such cap too. It is Intel vendor specific right now, but SIOV is
> > > public and other vendors may implement to the spec. Is there a good place to
> > > put the common capability check for that?
> > 
> > I'm still really unhappy with these SIOV caps. It was explained this
> > is just a hack to make up for pci_ims_array_create_msi_irq_domain()
> > succeeding in VM cases when it doesn't actually work.
> > 
> > Someday this is likely to get fixed, so tying platform behavior to PCI
> > caps is completely wrong.
> > 
> > This needs to be solved in the platform code,
> > pci_ims_array_create_msi_irq_domain() should not succeed in these
> > cases.
> 
> That sounds reasonable. Are you asking that the IMS cap check should gate
> the success/failure of pci_ims_array_create_msi_irq_domain() rather than the
> driver?

There shouldn't be an IMS cap at all

As I understand, the problem here is the only way to establish new
VT-d IRQ routing is by trapping and emulating MSI/MSI-X related
activities and triggering routing of the vectors into the guest.

There is a missing hypercall to allow the guest to do this on its own,
presumably it will someday be fixed so IMS can work in guests.

Until the hypercall is added pci_ims_array_create_msi_irq_domain()
should simply fail in guests. No PCI cap check required.

Jason