Re: [PATCH 00/10] KVM PCIe/MSI passthrough on ARM/ARM64

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 2016-01-29 at 15:35 +0100, Eric Auger wrote:
> Hi Alex,
> On 01/28/2016 10:51 PM, Alex Williamson wrote:
> > On Tue, 2016-01-26 at 13:12 +0000, Eric Auger wrote:
> > > This series addresses KVM PCIe passthrough with MSI enabled on ARM/ARM64.
> > > It pursues the efforts done on [1], [2], [3]. It also aims at covering the
> > > same need on some PowerPC platforms.
> > >  
> > > On x86 all accesses to the 1MB PA region [FEE0_0000h - FEF0_000h] are directed
> > > as interrupt messages: accesses to this special PA window directly target the
> > > APIC configuration space and not DRAM, meaning the downstream IOMMU is bypassed.
> > >  
> > > This is not the case on above mentionned platforms where MSI messages emitted
> > > by devices are conveyed through the IOMMU. This means an IOVA/host PA mapping
> > > must exist for the MSI to reach the MSI controller. Normal way to create
> > > IOVA bindings consists in using VFIO DMA MAP API. However in this case
> > > the MSI IOVA is not mapped onto guest RAM but on host physical page (the MSI
> > > controller frame).
> > >  
> > > Following first comments, the spirit of [2] is kept: the guest registers
> > > an IOVA range reserved for MSI mapping. When the VFIO-PCIe driver allocates
> > > its MSI vectors, it overwrites the MSI controller physical address with an IOVA,
> > > allocated within the window provided by the userspace. This IOVA is mapped
> > > onto the MSI controller frame physical page.
> > >  
> > > The series does not address yet the problematic of telling the userspace how
> > > much IOVA he should provision.
> > 
> > I'm sort of on a think-different approach today, so bear with me; how is
> > it that x86 can make interrupt remapping so transparent to drivers like
> > vfio-pci while for ARM and ppc we seem to be stuck with doing these
> > fixups of the physical vector ourselves, implying ugly (no offense)
> > paths bouncing through vfio to connect the driver and iommu backends?
> > 
> > We know that x86 handles MSI vectors specially, so there is some
> > hardware that helps the situation.  It's not just that x86 has a fixed
> > range for MSI, it's how it manages that range when interrupt remapping
> > hardware is enabled.  A device table indexed by source-ID references a
> > per device table indexed by data from the MSI write itself.  So we get
> > much, much finer granularity,
> About the granularity, I think ARM GICv3 now provides a similar
> capability with GICv3 ITS (interrupt translation service). Along with
> the MSI MSG write transaction, the device outputs a DeviceID conveyed on
> the bus. This DeviceID (~ your source-ID) enables to index a device
> table. The entry in the device table points to a DeviceId interrupt
> translation table indexed by the EventID found in the msi msg. So the
> entry in the interrupt translation table eventually gives you the
> eventual interrupt ID targeted by the MSI MSG.
> This translation capability if not available in GICv2M though, ie. the
> one I am currently using.
> 
> Those tables currently are built by the ITS irqchip (irq-gic-v3-its.c)

So it sounds like the interrupt remapping plumbing needs to be
implemented for those chips.  How does ITS identify an MSI versus any
other DMA write?  Does it need to be within a preconfigured address
space like on x86 or does it know this implicitly by the transaction
(which doesn't seem possible on PCIe)?

Along with this discussion, we should probably be revisiting whether
existing ARM SMMUs should be exposing the IOMMU_CAP_INTR_REMAP
capability.  This capability is meant to indicate interrupt isolation,
but if an entire page of IOVA space is mapped through the IOMMU to a
range of interrupts and some of those interrupts are shared with host
devices or other VMs, then we really don't have that isolation and the
system is susceptible to one VM interfering with another or with the
host.  If that's the case, the SMMU should not be claiming
IOMMU_CAP_INTR_REMAP.

>  but there's still effectively an interrupt
> > domain per device that's being transparently managed under the covers
> > whenever we request an MSI vector for a device.
> > 
> > So why can't we do something more like that here?  There's no predefined
> > MSI vector range, so defining an interface for the user to specify that
> > is unavoidable.
> Do you confirm that VFIO user API still still is the good choice to
> provide that IOVA range?

I don't see that we have an option there unless ARM wants to
retroactively reserve a range of IOVA space in the spec, which is
certainly not going to happen.  The only other thing that comes to mind
would be if there was an existing address space which could never be
backed by RAM or other DMA capable targets.  But that seems far fetched
as well.

>   But why shouldn't everything else be transparent?  We
> > could add an interface to the IOMMU API that allows us to register that
> > reserved range for the IOMMU domain.  IOMMU-core (or maybe interrupt
> > remapping) code might allocate an IOVA domain for this just as you've
> > done in the type1 code here.
> I have no objection to move that iova allocation scheme somewhere else.
> I just need to figure out how to deal with the fact iova.c is not
> compiled everywhere as I noticed too late ;-)
>   But rather than having any interaction
> > with vfio-pci, why not do this at lower levels such that the platform
> > interrupt vector allocation code automatically uses one of those IOVA
> > ranges and returns the IOVA rather than the physical address for the PCI
> > code to program into the device?  I think we know what needs to be done,
> > but we're taking the approach of managing the space ourselves and doing
> > a fixup of the device after the core code has done its job when we
> > really ought to be letting the core code manage a space that we define
> > and programming the device so that it doesn't need a fixup in the
> > vfio-pci code.  Wouldn't it be nicer if pci_enable_msix_range() returned
> > with the device properly programmed or generate an error if there's not
> > enough reserved mapping space in IOMMU domain?  Can it be done?
> I agree with you on the fact it would be cleaner to manage that natively
> at MSI controller level instead of patching the address value in
> vfio_pci_intrs.c. I will investigate in that direction but I need some
> more time to understand the links between the MSI controller, the PCI
> device and the IOMMU.

Since the current interrupt remapping schemes seem to operate in a
different address space, I expect there will be work to do to fit the
interrupt remapping within a provided address space, but it seems like a
very reasonable constraint to add.  Thanks,

Alex