Re: [PATCH] iommu: making IOMMU sysfs nodes API public

[Alex Williamson]

On Wed, 2013-02-20 at 15:20 +1100, Alexey Kardashevskiy wrote:
> On 20/02/13 14:47, Alex Williamson wrote:
> > On Wed, 2013-02-20 at 13:31 +1100, Alexey Kardashevskiy wrote:
> >> On 20/02/13 07:11, Alex Williamson wrote:
> >>> On Tue, 2013-02-19 at 18:38 +1100, David Gibson wrote:
> >>>> On Mon, Feb 18, 2013 at 10:24:00PM -0700, Alex Williamson wrote:
> >>>>> On Mon, 2013-02-18 at 17:15 +1100, Alexey Kardashevskiy wrote:
> >>>>>> On 13/02/13 04:15, Alex Williamson wrote:
> >>>>>>> On Wed, 2013-02-13 at 01:42 +1100, Alexey Kardashevskiy wrote:
> >>>>>>>> On 12/02/13 16:07, Alex Williamson wrote:
> >>>>>>>>> On Tue, 2013-02-12 at 15:06 +1100, Alexey Kardashevskiy wrote:
> >>>>>>>>>> Having this patch in a tree, adding new nodes in sysfs
> >>>>>>>>>> for IOMMU groups is going to be easier.
> >>>>>>>>>>
> >>>>>>>>>> The first candidate for this change is a "dma-window-size"
> >>>>>>>>>> property which tells a size of a DMA window of the specific
> >>>>>>>>>> IOMMU group which can be used later for locked pages accounting.
> >>>>>>>>>
> >>>>>>>>> I'm still churning on this one; I'm nervous this would basically creat
> >>>>>>>>> a /proc free-for-all under /sys/kernel/iommu_group/$GROUP/ where any
> >>>>>>>>> iommu driver can add random attributes.  That can get ugly for
> >>>>>>>>> userspace.
> >>>>>>>>
> >>>>>>>> Is not it exactly what sysfs is for (unlike /proc)? :)
> >>>>>>>
> >>>>>>> Um, I hope it's a little more thought out than /proc.
> >>>>>>>
> >>>>>>>>> On the other hand, for the application of userspace knowing how much
> >>>>>>>>> memory to lock for vfio use of a group, it's an appealing location to
> >>>>>>>>> get that information.  Something like libvirt would already be poking
> >>>>>>>>> around here to figure out which devices to bind.  Page limits need to be
> >>>>>>>>> setup prior to use through vfio, so sysfs is more convenient than
> >>>>>>>>> through vfio ioctls.
> >>>>>>>>
> >>>>>>>> True. DMA window properties do not change since boot so sysfs is the right
> >>>>>>>> place to expose them.
> >>>>>>>>
> >>>>>>>>> But then is dma-window-size just a vfio requirement leaking over into
> >>>>>>>>> iommu groups?  Can we allow iommu driver based attributes without giving
> >>>>>>>>> up control of the namespace?  Thanks,
> >>>>>>>>
> >>>>>>>> Who are you asking these questions? :)
> >>>>>>>
> >>>>>>> Anyone, including you.  Rather than dropping misc files in sysfs to
> >>>>>>> describe things about the group, I think the better solution in your
> >>>>>>> case might be a link from the group to an existing sysfs directory
> >>>>>>> describing the PE.  I believe your PE is rooted in a PCI bridge, so that
> >>>>>>> presumably already has a representation in sysfs.  Can the aperture size
> >>>>>>> be determined from something in sysfs for that bridge already?  I'm just
> >>>>>>> not ready to create a grab bag of sysfs entries for a group yet.
> >>>>>>> Thanks,
> >>>>>>
> >>>>>>
> >>>>>> At the moment there is no information neither in sysfs nor
> >>>>>> /proc/device-tree about the dma-window. And adding a sysfs entry per PE
> >>>>>> (powerpc partitionable end-point which is often a PHB but not always) just
> >>>>>> for VFIO is quite heavy.
> >>>>>
> >>>>> How do you learn the window size and PE extents in the host kernel?
> >>>>>
> >>>>>> We could add a ppc64 subfolder under /sys/kernel/iommu/xxx/ and put the
> >>>>>> "dma-window" property there. And replace it with a symlink when and if we
> >>>>>> add something for PE later. Would work?
> >>>>
> >>>> Fwiw, I'd suggest a subfolder named for the type of IOMMU, rather than
> >>>> "ppc64".
> >>>>
> >>>>> To be clear, you're suggesting /sys/kernel/iommu_groups/$GROUP/xxx/,
> >>>>> right?  A subfolder really only limits the scope of the mess, so it's
> >>>>> not much improvement.  What does the interface look like to make those
> >>>>> subfolders?
> >>>>>
> >>>>> The problem we're trying to solve is this call flow:
> >>>>>
> >>>>> containerfd = open("/dev/vfio/vfio");
> >>>>> ioctl(containerfd, VFIO_GET_API_VERSION);
> >>>>> ioctl(containerfd, VFIO_CHECK_EXTENSION, ...);
> >>>>> groupfd = open("/dev/vfio/$GROUP");
> >>>>> ioctl(groupfd, VFIO_GROUP_GET_STATUS);
> >>>>> ioctl(groupfd, VFIO_GROUP_SET_CONTAINER, &containerfd);
> >>>>>
> >>>>> You wanted to lock all the memory for the DMA window here, before we can
> >>>>> call VFIO_IOMMU_GET_INFO, but does it need to happen there?  We still
> >>>>> have a MAP_DMA hook.  We could do it all on the first mapping.
> >>>>
> >>>> MAP_DMA isn't quite enough, since the guest can also directly cause
> >>>> mappings using hypercalls directly implemented in KVM.  I think it
> >>>> would be feasible to lock on the first mapping (either via MAP_DMA, or
> >>>> H_PUT_TCE) though it would be a bit ugly and require that the first
> >>>> H_PUT_TCE always bounce out to virtual mode (Alexey, correct me if I'm
> >>>> wrong here).  IIRC there is also a call to bind the vfio container to
> >>>> a (qemu assigned) LIOBN, before the guest can use H_PUT_TCE directly,
> >>>> so that might be another place we could do the lock.
> >>>
> >>> Somehow hypercall mappings have to be gated by the userspace setup,
> >>> right?
> >>
> >>
> >> There is a KVM ioctl (and a KVM capability) which hooks LIOBN (PCI bus ID)
> >> with IOMMU ID. It basically creates an entry in the list of all LIOBNs and
> >> when TCE call occurs, the host finds correct IOMMU group to pass this call to.
> >>
> >> It happens from spapr_register_vfio_container() in QEMU, i.e. after getting
> >> DMA window properties but only if the host supports real mode TCE handling.
> >>
> >>
> >>>>>    It also
> >>>>> has a flags field that could augment the behavior to trigger page
> >>>>> locking.
> >>>>
> >>>> I don't see how the flags help us - we can't have userspace choose to
> >>>> skip the locked memory accounting.  Or are you suggesting a flag to
> >>>> open the container in some sort of dummy mode where only GET_INFO is
> >>>> possible, then re-open with the full locking?
> >>>
> >>> Sort of, I don't think it needs to be re-opened, but we had previously
> >>> talked about some kind of enable and disable ioctl.  "enable" would be
> >>> the logical place to lock pages, but then we probably got stuck in
> >>> questions around what it means to enable an iommu generically.
> >>
> >> The other question is if a container is ready to work if I add just one
> >> group? What happens when I add another one (not supported on ppc64 but
> >> still)?
> >
> > This is also the problem with exposing a dma window under the group in
> > sysfs.  Do I require the ability to lock the sum of the window, the
> > largest window, what?  If we rely on the ioctls, userspace can figure
> > out that they can't be combined and know it's the sum.  I'm not sure
> > what your plans are around hotplug of a PHB though.
> >
> >> Having "enable" method and disabling new attachments when it is
> >> "enabled" would keep my brain calm :)
> >
> > Now I'm not sure whether you're for or against it ;)
> 
> 
> I am for introducing enable() ioctls :) Or even "lock" ioctl.
> 
> 
> >>> So what
> >>> if instead of a separate enable ioctl we had a flag on DMA_MAP that was
> >>> defined as SET_WINDOW where iova and size are passed and specify the
> >>> portion of the DMA window that userspace intends to use and which is
> >>> therefore locked.  If you don't support subwindows, fine, just fail it.
> >>> You could have a matching PUT_WINDOW on DMA_UNMAP if you wanted.
> >>
> >> DMA_MAP which does not do "map" but does "lock" or "set window"?
> >> enable()/disable() look better.
> >
> > Sure, this is why we have a modular iommu interface, spapr can create an
> > enable ioctl if necessary.  I think there are ways to use the
> > DMA_MAP/UNMAP ioctl in ways that aren't a complete kludge though.
> >
> >>>>>    Adding the window size to sysfs seems more readily convenient,
> >>>>> but is it so hard for userspace to open the files and call a couple
> >>>>> ioctls to get far enough to call IOMMU_GET_INFO?  I'm unconvinced the
> >>>>> clutter in sysfs more than just a quick fix.  Thanks,
> >>>>
> >>>> And finally, as Alexey points out, isn't the point here so we know how
> >>>> much rlimit to give qemu?  Using ioctls we'd need a special tool just
> >>>> to check the dma window sizes, which seems a bit hideous.
> >>>
> >>> Is it more hideous that using iommu groups to report a vfio imposed
> >>> restriction?  Are a couple open files and a handful of ioctls worse than
> >>> code to parse directory entries and the future maintenance of an
> >>> unrestricted grab bag of sysfs entries?
> >>
> >> At the moment DMA32 window properties are static. So I can easily get rid
> >> of VFIO_IOMMU_SPAPR_TCE_GET_INFO and be happy.
> >
> > Like, for instance, every PE always gets 512MB DMA window, fixed base
> > address, not configurable, end of story?
> 
> 
> Almost :) 1GB, starting at 0 (sometime at 2GB). Multiple PCI domains are 
> supported on ppc64 so it does not make a problem as bus address spaces are 
> separated. But yes, not flexible at all.

Statements like "at the moment...", "[but] sometimes at..." make me
think it's best to keep the GET_INFO call.

> >> Ah, anyway, how do you see these ioctls to work on a user machine?
> >> A separate tool which takes an iommu id, returns DMA window size and
> >> adjusts rlimit?
> >
> > Sure, we need something that provides the function of libvirt and
> > unbinds devices from host drivers, re-binds them to vfio-pci.  That tool
> > needs to have permissions to manipulate groups, so we're just talking
> > about whether it's stepping over the line for it to open the group and a
> > container, associate them, and probe the iommu info vs reading a sysfs
> > file.  Thanks,
> 
> So the Tool is going to be a part of libvirt but not kernel or qemu, right?
> Then implementing "LOCK" (and call it after GET_INFO in QEMU and not call 
> it from the Tool) should work fine.

Right, a probe tool would check the value, close the files and set the
locked page limit for qemu, which would take the next step to trigger
the in-kernel accounting.  Thanks,

Alex

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/