Re: [RFC PATCH v6 04/20] iommu/arm-smmu: add capability IOMMU_CAP_INTR_REMAP

Alex Williamson
2014-06-26T19:36:24Z

On Thu, 2014-06-26 at 19:10 +0000, Chalamarla, Tirumalesh wrote:
> Thanks for the clarification Alex, That’s exactly my point, why are we relying on QEMU or something else to emulate the MSI space when we can directly give access to devices using ITS (of course with a small emulation code).
> This way we are also benefited from all ITS services like VCPU migration etc.

I have no idea what ITS is.

> What about non QEMU VFIO users, for example, if I wanted to use VFIO to assign a device to a user process I don't need to depend on QEMU. I thought this is one of the main goals of vfio to make it independent of hypervisors.

Where did QEMU become a requirement? Maybe I'm missing something in the
ARM part of the conversation that got chopped off, but this is exactly
why we have the VFIO/QEMU split that we do. VFIO provides basic
virtualization for config space and restricts access to other areas that
users shouldn't be allowed to change. QEMU is just one example of a
userspace VFIO driver. QEMU takes the decomposed device exposed through
the VFIO ABI and re-creates a PCI device out of it. VFIO itself has no
dependency on QEMU. Thanks,

Alex

> -----Original Message-----
> From: Alex Williamson [mailto:alex.williamson@redhat.com]
> Sent: Thursday, June 26, 2014 12:00 PM
> To: Chalamarla, Tirumalesh
> Cc: Joerg Roedel; Will Deacon; kvm@vger.kernel.org; open list; stuart.yoder@freescale.com; iommu@lists.linux-foundation.org; tech@virtualopensystems.com; kvmarm@lists.cs.columbia.edu; moderated list:ARM SMMU DRIVER
> Subject: Re: [RFC PATCH v6 04/20] iommu/arm-smmu: add capability IOMMU_CAP_INTR_REMAP
>
> On Thu, 2014-06-26 at 18:41 +0000, Chalamarla, Tirumalesh wrote:
> > Sorry there was a type,
> >
> > The question is:
> >
> > How is VFIO restricting software from writing to MSI/MSI-X vectors of the device.
>
> All interrupts are configured via ioctl, not MSI config space or the MSI-X vector table in MMIO space. VFIO protects the MSI config area by virtualizing it (you can't actually write the physical enable bit or address/data through VFIO). The MSI-X vector table is protected by preventing read, write, or mmap access to it. QEMU provides further virtualization above the basics provided by VFIO. We really can't guarantee that devices don't have backdoors to configure these though.
> See the realtek quirk in QEMU for an example of a device that has such a backdoor. That's why we require interrupt remapping, so that a device that does this can only hurt the guest, and require the user to opt-out if they feel they have a sufficiently trusted guest. Thanks,
>
> Alex
>
> >
> > -----Original Message-----
> > From: Chalamarla, Tirumalesh
> > Sent: Thursday, June 26, 2014 11:16 AM
> > To: Chalamarla, Tirumalesh; Joerg Roedel; Will Deacon
> > Cc: kvm@vger.kernel.org; open list; alex.williamson@redhat.com;
> > stuart.yoder@freescale.com; iommu@lists.linux-foundation.org;
> > tech@virtualopensystems.com; kvmarm@lists.cs.columbia.edu; moderated
> > list:ARM SMMU DRIVER
> > Subject: RE: [RFC PATCH v6 04/20] iommu/arm-smmu: add capability
> > IOMMU_CAP_INTR_REMAP
> >
> > When I say emulating ITS, I mean translating guest ITS commands to physical ITS commands and placing them in physical queue.
> >
> > Regards,
> > Tirumalesh.
> >
> > -----Original Message-----
> > From: kvmarm-bounces@lists.cs.columbia.edu
> > [mailto:kvmarm-bounces@lists.cs.columbia.edu] On Behalf Of Chalamarla,
> > Tirumalesh
> > Sent: Thursday, June 26, 2014 11:08 AM
> > To: Joerg Roedel; Will Deacon
> > Cc: kvm@vger.kernel.org; open list; alex.williamson@redhat.com;
> > stuart.yoder@freescale.com; iommu@lists.linux-foundation.org;
> > tech@virtualopensystems.com; kvmarm@lists.cs.columbia.edu; moderated
> > list:ARM SMMU DRIVER
> > Subject: RE: [RFC PATCH v6 04/20] iommu/arm-smmu: add capability
> > IOMMU_CAP_INTR_REMAP
> >
> > Forgive me if this discussion is not relative here, but I thought it is.
> >
> > How is VFIO restricting devices from writing to MSI/MSI-X, Is all the vector area is mapped by VFIO to trap the accesses. I am asking this because we might need to emulate ITS somewhere either in KVM or VFIO to provide direct access to devices.
> > And I don't see any mentions on that. I think this flag needs to be set by ITS emulation.
> >
> > Regards,
> > Tirumalesh.
> >
> > -----Original Message-----
> > From: kvmarm-bounces@lists.cs.columbia.edu
> > [mailto:kvmarm-bounces@lists.cs.columbia.edu] On Behalf Of Joerg
> > Roedel
> > Sent: Monday, June 16, 2014 8:39 AM
> > To: Will Deacon
> > Cc: stuart.yoder@freescale.com; kvm@vger.kernel.org; open list;
> > iommu@lists.linux-foundation.org; alex.williamson@redhat.com;
> > moderated list:ARM SMMU DRIVER; tech@virtualopensystems.com;
> > kvmarm@lists.cs.columbia.edu; Christoffer Dall
> > Subject: Re: [RFC PATCH v6 04/20] iommu/arm-smmu: add capability
> > IOMMU_CAP_INTR_REMAP
> >
> > On Mon, Jun 16, 2014 at 04:25:26PM +0100, Will Deacon wrote:
> > > Ok, thanks. In which case, I think this is really a combined
> > > property of the SMMU and the interrupt controller, so we might need
> > > some extra code so that the SMMU can check that the interrupt
> > > controller for the device is also capable of interrupt remapping.
> >
> > Right, that this is part of IOMMU code has more or less historic reasons on x86. Interrupt remapping is purely implemented in the IOMMU there, so on ARM some clue-code between interrupt controler and smmu is needed.
> >
> >
> > Joerg
> >
> >
> > _______________________________________________
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> > kvmarm@lists.cs.columbia.edu
> > https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
> > _______________________________________________
> > kvmarm mailing list
> > kvmarm@lists.cs.columbia.edu
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>
>
>

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