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Hi! At redhat we've been thinking about how to support vGPUs from multiple vendors in a common way within QEMU. We want to enable code sharing between vendors and give new vendors an easy path to add their own support. We also have the complication that not all vGPU vendors are as open source friendly as Intel, so being able to abstract the device mediation and access outside of QEMU is a big advantage. The proposal I'd like to make is that a vGPU, whether it is from Intel or another vendor, is predominantly a PCI(e) device. We have an interface in QEMU already for exposing arbitrary PCI devices, vfio-pci. Currently vfio-pci uses the VFIO API to interact with "physical" devices and system IOMMUs. I highlight /physical/ there because some of these physical devices are SR-IOV VFs, which is somewhat of a fuzzy concept, somewhere between fixed hardware and a virtual device implemented in software. That software just happens to be running on the physical endpoint.

Agree. One clarification for rest discussion, is that we're talking about GVT-g vGPU here which is a pure software GPU virtualization technique. GVT-d (note some use in the text) refers to passing through the whole GPU or a specific VF. GVT-d already falls into existing VFIO APIs nicely (though some on-going effort to remove Intel specific platform stickness from gfx driver). :-)

vGPUs are similar, with the virtual device created at a different point, host software. They also rely on different IOMMU constructs, making use of the MMU capabilities of the GPU (GTTs and such), but really having similar requirements.

One important difference between system IOMMU and GPU-MMU here. System IOMMU is very much about translation from a DMA target (IOVA on native, or GPA in virtualization case) to HPA. However GPU internal MMUs is to translate from Graphics Memory Address (GMA) to DMA target (HPA if system IOMMU is disabled, or IOVA/GPA if system IOMMU is enabled). GMA is an internal addr space within GPU, not exposed to Qemu and fully managed by GVT-g device model. Since it's not a standard PCI defined resource, we don't need abstract this capability in VFIO interface.

The proposal is therefore that GPU vendors can expose vGPUs to userspace, and thus to QEMU, using the VFIO API. For instance, vfio supports modular bus drivers and IOMMU drivers. An intel-vfio-gvt-d module (or extension of i915) can register as a vfio bus driver, create a struct device per vGPU, create an IOMMU group for that device, and register that device with the vfio-core. Since we don't rely on the system IOMMU for GVT-d vGPU assignment, another vGPU vendor driver (or extension of the same module) can register a "type1" compliant IOMMU driver into vfio-core. From the perspective of QEMU then, all of the existing vfio-pci code is re-used, QEMU remains largely unaware of any specifics of the vGPU being assigned, and the only necessary change so far is how QEMU traverses sysfs to find the device and thus the IOMMU group leading to the vfio group.

GVT-g requires to pin guest memory and query GPA->HPA information, upon which shadow GTTs will be updated accordingly from (GMA->GPA) to (GMA->HPA). So yes, here a dummy or simple "type1" compliant IOMMU can be introduced just for this requirement. However there's one tricky point which I'm not sure whether overall VFIO concept will be violated. GVT-g doesn't require system IOMMU to function, however host system may enable system IOMMU just for hardening purpose. This means two-level translations existing (GMA-> IOVA->HPA), so the dummy IOMMU driver has to request system IOMMU driver to allocate IOVA for VMs and then setup IOVA->HPA mapping in IOMMU page table. In this case, multiple VM's translations are multiplexed in one IOMMU page table. We might need create some group/sub-group or parent/child concepts among those IOMMUs for thorough permission control.

There are a few areas where we know we'll need to extend the VFIO API to make this work, but it seems like they can all be done generically. One is that PCI BARs are described through the VFIO API as regions and each region has a single flag describing whether mmap (ie. direct mapping) of that region is possible. We expect that vGPUs likely need finer granularity, enabling some areas within a BAR to be trapped and fowarded as a read or write access for the vGPU-vfio-device module to emulate, while other regions, like framebuffers or texture regions, are directly mapped. I have prototype code to enable this already.

Yes in GVT-g one BAR resource might be partitioned among multiple vGPUs. If VFIO can support such partial resource assignment, it'd be great. Similar parent/child concept might also be required here, so any resource enumerated on a vGPU shouldn't break limitations enforced on the physical device. One unique requirement for GVT-g here, though, is that vGPU device model need to know guest BAR configuration for proper emulation (e.g. register IO emulation handler to KVM). Similar is about guest MSI vector for virtual interrupt injection. Not sure how this can be fit into common VFIO model. Does VFIO allow vendor specific extension today?

Another area is that we really don't want to proliferate each vGPU needing a new IOMMU type within vfio. The existing type1 IOMMU provides potentially the most simple mapping and unmapping interface possible. We'd therefore need to allow multiple "type1" IOMMU drivers for vfio, making type1 be more of an interface specification rather than a single implementation. This is a trivial change to make within vfio and one that I believe is compatible with the existing API. Note that implementing a type1-compliant vfio IOMMU does not imply pinning an mapping every registered page. A vGPU, with mediated device access, may use this only to track the current HVA to GPA mappings for a VM. Only when a DMA is enabled for the vGPU instance is that HVA pinned and an HPA to GPA translation programmed into the GPU MMU. Another area of extension is how to expose a framebuffer to QEMU for seamless integration into a SPICE/VNC channel. For this I believe we could use a new region, much like we've done to expose VGA access through a vfio device file descriptor. An area within this new framebuffer region could be directly mappable in QEMU while a non-mappable page, at a standard location with standardized format, provides a description of framebuffer and potentially even a communication channel to synchronize framebuffer captures. This would be new code for QEMU, but something we could share among all vGPU implementations.

Now GVT-g already provides an interface to decode framebuffer information, w/ an assumption that the framebuffer will be further composited into OpenGL APIs. So the format is defined according to OpenGL definition. Does that meet SPICE requirement? Another thing to be added. Framebuffers are frequently switched in reality. So either Qemu needs to poll or a notification mechanism is required. And since it's dynamic, having framebuffer page directly exposed in the new region might be tricky. We can just expose framebuffer information (including base, format, etc.) and let Qemu to map separately out of VFIO interface. And... this works fine with vGPU model since software knows all the detail about framebuffer. However in pass-through case, who do you expect to provide that information? Is it OK to introduce vGPU specific APIs in VFIO?

Another obvious area to be standardized would be how to discover, create, and destroy vGPU instances. SR-IOV has a standard mechanism to create VFs in sysfs and I would propose that vGPU vendors try to standardize on similar interfaces to enable libvirt to easily discover the vGPU capabilities of a given GPU and manage the lifecycle of a vGPU instance.

Now there is no standard. We expose vGPU life-cycle mgmt. APIs through sysfs (under i915 node), which is very Intel specific. In reality different vendors have quite different capabilities for their own vGPUs, so not sure how standard we can define such a mechanism. But this code should be minor to be maintained in libvirt.

This is obviously a lot to digest, but I'd certainly be interested in hearing feedback on this proposal as well as try to clarify anything I've left out or misrepresented above. Another benefit to this mechanism is that direct GPU assignment and vGPU assignment use the same code within QEMU and same API to the kernel, which should make debugging and code support between the two easier. I'd really like to start a discussion around this proposal, and of course the first open source implementation of this sort of model will really help to drive the direction it takes. Thanks!

Thanks for starting this discussion. Intel will definitely work with community on this work. Based on earlier comments, I'm not sure whether we can exactly same code for direct GPU assignment and vGPU assignment, since even we extend VFIO some interfaces might be vGPU specific. Does this way still achieve your end goal? Thanks Kevin

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Re: [Xen-devel] [Intel-gfx] [Announcement] 2015-Q3 release of XenGT - a Mediated Graphics Passthrough Solution from Intel