Re: [PATCH RFC 0/8] basic vfio-ccw infrastructure

Alex Williamson
2016-05-05T19:19:45Z

[cc +Intel,NVIDIA]

On Thu, 5 May 2016 18:29:08 +0800
Dong Jia <bjsdjshi@linux.vnet.ibm.com> wrote:

> On Wed, 4 May 2016 13:26:53 -0600
> Alex Williamson <alex.williamson@redhat.com> wrote:
>
> > On Wed, 4 May 2016 17:26:29 +0800
> > Dong Jia <bjsdjshi@linux.vnet.ibm.com> wrote:
> >
> > > On Fri, 29 Apr 2016 11:17:35 -0600
> > > Alex Williamson <alex.williamson@redhat.com> wrote:
> > >
> > > Dear Alex:
> > >
> > > Thanks for the comments.
> > >
> > > [...]
> > >
> > > > >
> > > > > The user of vfio-ccw is not limited to Qemu, while Qemu is definitely a
> > > > > good example to get understand how these patches work. Here is a little
> > > > > bit more detail how an I/O request triggered by the Qemu guest will be
> > > > > handled (without error handling).
> > > > >
> > > > > Explanation:
> > > > > Q1-Q4: Qemu side process.
> > > > > K1-K6: Kernel side process.
> > > > >
> > > > > Q1. Intercept a ssch instruction.
> > > > > Q2. Translate the guest ccw program to a user space ccw program
> > > > > (u_ccwchain).
> > > >
> > > > Is this replacing guest physical address in the program with QEMU
> > > > virtual addresses?
> > > Yes.
> > >
> > > >
> > > > > Q3. Call VFIO_DEVICE_CCW_CMD_REQUEST (u_ccwchain, orb, irb).
> > > > > K1. Copy from u_ccwchain to kernel (k_ccwchain).
> > > > > K2. Translate the user space ccw program to a kernel space ccw
> > > > > program, which becomes runnable for a real device.
> > > >
> > > > And here we translate and likely pin QEMU virtual address to physical
> > > > addresses to further modify the program sent into the channel?
> > > Yes. Exactly.
> > >
> > > >
> > > > > K3. With the necessary information contained in the orb passed in
> > > > > by Qemu, issue the k_ccwchain to the device, and wait event q
> > > > > for the I/O result.
> > > > > K4. Interrupt handler gets the I/O result, and wakes up the wait q.
> > > > > K5. CMD_REQUEST ioctl gets the I/O result, and uses the result to
> > > > > update the user space irb.
> > > > > K6. Copy irb and scsw back to user space.
> > > > > Q4. Update the irb for the guest.
> > > >
> > > > If the answers to my questions above are both yes,
> > > Yes, they are.
> > >
> > > > then this is really a mediated interface, not a direct assignment.
> > > Right. This is true.
> > >
> > > > We don't need an iommu
> > > > because we're policing and translating the program for the device
> > > > before it gets sent to hardware. I think there are better ways than
> > > > noiommu to handle such devices perhaps even with better performance
> > > > than this two-stage translation. In fact, I think the solution we plan
> > > > to implement for vGPU support would work here.
> > > >
> > > > Like your device, a vGPU is mediated, we don't have IOMMU level
> > > > translation or isolation since a vGPU is largely a software construct,
> > > > but we do have software policing and translating how the GPU is
> > > > programmed. To do this we're creating a type1 compatible vfio iommu
> > > > backend that uses the existing map and unmap ioctls, but rather than
> > > > programming them into an IOMMU for a device, it simply stores the
> > > > translations for use by later requests. This means that a device
> > > > programmed in a VM with guest physical addresses can have the
> > > > vfio kernel convert that address to process virtual address, pin the
> > > > page and program the hardware with the host physical address in one
> > > > step.
> > > I've read through the mail threads those discuss how to add vGPU
> > > support in VFIO. I'm afraid that proposal could not be simply addressed
> > > to this case, especially if we want to make the vfio api completely
> > > compatible with the existing usage.
> > >
> > > AFAIU, a PCI device (or a vGPU device) uses a dedicated, exclusive and
> > > fixed range of address in the memory space for DMA operations. Any
> > > address inside this range will not be used for other purpose. Thus we
> > > can add memory listener on this range, and pin the pages for further
> > > use (DMA operation). And we can keep the pages pinned during the life
> > > cycle of the VM (not quite accurate, or I should say 'the target
> > > device').
> >
> > That's not entirely accurate. Ignoring a guest IOMMU, current device
> > assignment pins all of guest memory, not just a dedicated, exclusive
> > range of it, in order to map it through the hardware IOMMU. That gives
> > the guest the ability to transparently perform DMA with the device
> > since the IOMMU maps the guest physical to host physical translations.
> Thanks for this explanation.
>
> I noticed in the Qemu part, when we tried to introduce vfio-pci to the
> s390 architecture, we set the IOMMU width by calling
> memory_region_add_subregion before initializing the address_space of
> the PCI device, which will be registered with the vfio_memory_listener
> later. The 'width' of the subregion is what I called the 'range' in the
> former reply.
>
> The first reason we did that is, we know exactly the dma memory
> range, and we got the width by 'dma_addr_end - dma_addr_start'. The
> second reason we have to do that is, using the following statement will
> cause the initialization of the guest tremendously long:
> group = vfio_get_group(groupid, &address_space_memory);
> Because doing map on [0, UINT64_MAX] range does cost lots of time. For
> me, it's unacceptably long (more than 5 minutes).
>
> My questions are:
> 1. Why we have to 'pin all of guest memory' if we do know the
> iommu memory range?

We have a few different configuration here, so let's not confuse them.
On x86 with pci device assignment we typically don't have a guest IOMMU
so the guest assumes the device can DMA to any address in the guest
memory space. To enable that we pin all of guest memory and map it
through the IOMMU. Even with a guest IOMMU on x86, it's an optional
feature that the guest OS may or may not use, so we'll always at least
startup in this mode and the guest may or may not enable something else.

When we have a guest IOMMU available, the device switches to a
different address space, note that in current QEMU code,
vfio_get_group() is actually called as:

group = vfio_get_group(groupid, pci_device_iommu_address_space(pdev));

Where pci_device_iommu_address_space() determines whether the device is
translated by an IOMMU and defaults back to &address_space_memory if
not. So we already have code that is supposed to handle the difference
between whether we're mapping all of guest memory or whether we're only
registering translations populated in the IOMMU for the device.

It appears that S390 implements some sort of IOMMU in the guest, so
theoretically DMA_MAP and DMA_UNMAP operations are only going to map
the IOTLB translations relevant to that device. At least that's how
it's supposed to work. So we shouldn't be pinning all of guest memory
for the PCI case.

When we switch to the vgpu/mediated-device approach, everything should
work the same except the DMA_MAP and DMA_UNMAP ioctls don't do any
pinning or IOMMU mapping. They only update the in-kernel vfio view of
IOVA to process virtual translations. These translations are then
consumed only when a device operation requires DMA. At that point we
do an IOVA-to-VA translation and page_to_pfn(get_user_pages()) to get a
host physical address, which is only pinned while the operation is
inflight.

> 2. Didn't you have the long time starting problem either? Or I
> must miss something. For the vfio-ccw case, there is no fixed range. So
> according to your proposal, vfio-ccw has to pin all of guest memory.
> And I guess I will encounter this problem again.

x86 with a guest IOMMU is very new and still not upstream, so I don't
know if there's a point at which we perform an operation over the
entire address space, that would be slow. It seems like something we
could optimize though. x86 without a guest IOMMU only performs DMA_MAP
operations for the actual populated guest memory. This is of course
not free, but is negligible for small guests and scales as the memory
size of the guest increases. According the the vgpu/mediated-device
proposal, there would be no pinning occurring at startup, the DMA_MAP
would only be populating a tree of IOVA-to-VA mappings using the
granularity of the DMA_MAP parameters itself.

> >
> > That's not what vGPU is about. In the case of vGPU the proposal is to
> > use the same QEMU vfio MemoryListener API, but only for the purpose of
> > having an accurate database of guest physical to process virtual
> > translations for the VM. In your above example, this means step Q2 is
> > eliminated because step K2 has the information to perform both a guest
> > physical to process virtual translation and to pin the page to get a
> > host physical address. So you'd only need to modify the program once.
> According to my understanding of your proposal, I should do:
> ------------------------------------------------------------
> #1. Introduce a vfio_iommu_type1_ccw as the vfio iommu backend for ccw.
> When starting the guest, pin all of guest memory, and form the database.

I hope that we can have a common type1-compatible iommu backend for
vfio, there's nothing ccw specific there. Pages would not be pinned,
only registered for later retrieval by the mediated-device backend and
only for the runtime of the ccw program in your case.

> #2. In the driver of the ccw devices, when an I/O instruction was
> intercepted, query the database and translate the ccw program for I/O
> operation.

The database query would be the point at which the page is pinned, so
there would be some sort of 'put' of the translation after the ccw
program executes to release the pin.

> I also noticed in another thread:
> ---------------------------------
> [Qemu-devel] [RFC PATCH v3 3/3] VFIO Type1 IOMMU change: to support with iommu and without iommu
>
> Kirti did:
> 1. don't pin the pages in the map ioctl for the vGPU case.
> 2. export vfio_pin_pages and vfio_unpin_pages.
>
> Although their patches didn't show how these interfaces were used, I
> guess them can either use these interfaces to pin/unpin all of the
> guest memory, or pin/unpin memory on demand. So can I reuse their work
> to finish my #1? If the answer is yes, then I could change my plan and

Yes, we would absolutely only want one vfio iommu backend doing this,
there's nothing device specific about it. We're looking at supporting
both modes of operation, fully pinned and pin-on-demand. NVIDIA vGPU
wants the on-demand approach while Intel vGPU wants to pin the entire
guest, at least for an initial solution. This iommu backend would need
to support both as determined by the mediated device backend.

> do:
> #1. Introduce a vfio_iommu_type1_ccw as the vfio iommu backend for ccw.
> When starting the guest, form the <vaddr, iova, size> database.
>
> #2. In the driver of the ccw devices, when an I/O instruction was
> intercepted, call vfio_pin_pages (Kirti's version) to get the host
> physical address, then translate the ccw program for I/O operation.
>
> So which one is the right way to go?

As above, I think we have a need to support both approaches in this new
iommu backend, it will be up to you to determine which is appropriate
for your devices and guest drivers. A fully pinned guest has a latency
advantage, but obviously there are numerous disadvantages for the
pinning itself. Pinning on-demand has overhead to setup each DMA
operations by the device but has a much smaller pinning footprint.

> > > Well, a Subchannel Device does not have such a range of address. The
> > > device driver simply calls kalloc() to get a piece of memory, and
> > > assembles a ccw program with it, before issuing the ccw program to
> > > perform an I/O operation. So the Qemu memory listener can't tell if an
> > > address is for an I/O operation, or for whatever else. And this makes
> > > the memory listener unnecessary for our case.
> >
> > It's only unnecessary because QEMU is manipulating the program to
> > replace those addresses with process virtual addresses. The purpose
> > of the MemoryListener in the vGPU approach is only to inform the
> > kernel so that it can perform that translation itself.
> >
> > > The only time point that we know we should pin pages for I/O, is the
> > > time that an I/O instruction (e.g. ssch) was intercepted. At this
> > > point, we know the address contented in the parameter of the ssch
> > > instruction points to a piece of memory that contents a ccw program.
> > > Then we do: pin the pages --> convert the ccw program --> perform the
> > > I/O --> return the I/O result --> and unpin the pages.
> >
> > And you could do exactly the same with the vGPU model, it's simply a
> > difference of how many times the program is converted and using the
> > MemoryListener to update guest physical to process virtual addresses in
> > the kernel.
> Understand.
>
> >
> > > > This architecture also makes the vfio api completely compatible with
> > > > existing usage without tainting QEMU with support for noiommu devices.
> > > > I would strongly suggest following a similar approach and dropping the
> > > > noiommu interface. We really do not need to confuse users with noiommu
> > > > devices that are safe and assignable and devices where noiommu should
> > > > warn them to stay away. Thanks,
> > > Understand. But like explained above, even if we introduce a new vfio
> > > iommu backend, what it does would probably look quite like what the
> > > no-iommu backend does. Any idea about this?
> >
> > It's not, a mediated device simply shifts the isolation guarantees from
> > hardware protection in an IOMMU to software protection in a mediated
> > vfio bus driver. The IOMMU interface simply becomes a database through
> > which we can perform in-kernel translations. All you want is the vfio
> > device model and you have the ability to do that in a secure way, which
> > is the same as vGPU. The no-iommu code is intended to provide the vfio
> > device model in a known-to-be-insecure means. I don't think you want
> > to build on that and I don't think we want no-iommu anywhere near
> > QEMU. Thanks,
> Got it. I will mimic the vGPU model, once the above questions are
> clarified. :>

Thanks,
Alex