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

From: Dong Jia
Subject: Re: [Qemu-devel] [PATCH RFC 0/8] basic vfio-ccw infrastructure
Date: Thu, 5 May 2016 18:29:08 +0800

On Wed, 4 May 2016 13:26:53 -0600
Alex Williamson <address@hidden> wrote:

> On Wed, 4 May 2016 17:26:29 +0800
> Dong Jia <address@hidden> wrote:
> > On Fri, 29 Apr 2016 11:17:35 -0600
> > Alex Williamson <address@hidden> 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?
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.

> 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.

#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

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
#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?

> > 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.

> > > 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. :>

> Alex

Dong Jia

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