Message-ID: <51468FC9.2030203@ozlabs.ru>
Date:	Mon, 18 Mar 2013 14:53:45 +1100
From:	Alexey Kardashevskiy <aik@...abs.ru>
To:	Alex Williamson <alex.williamson@...hat.com>
CC:	David Gibson <david@...son.dropbear.id.au>,
	Joerg Roedel <joerg.roedel@....com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu: making IOMMU sysfs nodes API public

On 20/02/13 15:33, Alex Williamson wrote:
> 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,

Continuing the discussion :)
In meanwhile I added/tested VFIO_IOMMU_ENABLE and VFIO_IOMMU_DISABLE like 
that (will repost the patch later, may be this week, only few changes there):


+       case VFIO_IOMMU_ENABLE: {
+               mutex_lock(&container->lock);
+               ret = tce_iommu_enable(container);
+               mutex_unlock(&container->lock);
+
+               return ret;
+       }
+       case VFIO_IOMMU_DISABLE: {
+               mutex_lock(&container->lock);
+               tce_iommu_disable(container);
+               mutex_unlock(&container->lock);
+
+               return 0;
+       }

and defined them as (not arch specific):

+/* IOCTLs to enable/disable IOMMU container usage */
+#define VFIO_IOMMU_ENABLE      _IO(VFIO_TYPE, VFIO_BASE + 15)
+#define VFIO_IOMMU_DISABLE     _IO(VFIO_TYPE, VFIO_BASE + 16)


should be ok, right?


There is another question. It is possible to compile
vfio_iommu_spapr_tce as a module. How/when is it supposed to be loaded?
A user may not want to do "modprobe vfio_iommu_spapr_tce" manually.


-- 
Alexey
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