XPDS13: Zero-copy display of guest framebuffers using GEM - John Baboval, Citrix
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The document discusses the implementation of zero-copy display of guest framebuffers in virtualization through the GEM (Graphics Execution Manager) framework. It details the complexities of GPU integration, the challenges of managing foreign pages, and the use of manual m2p_override to optimize framebuffer access. The proposal includes creating foreign-backed GEM objects and introducing new IOCTL for enhanced graphics handling in Xen-based virtualization environments.
XPDS13: Zero-copy display of guest framebuffers using GEM - John Baboval, Citrix
- 1. Zero-‐Copy Display of Guest Framebuffers using GEM John Baboval Citrix Systems, Inc. October 24th, 2013
- 2. Agenda • Xen video basics • ExisNng OpNmizaNons • GEM October 24th, 2013 Citrix Systems, Inc 2
- 3. Overview of QEMU Graphics (The quick version) QEMU Video Device Driver VM HW UI (device specific) (device independent) I/O port handlers dpy_update() • Called by HW when Linear Framebuffer scanlines change gfx_hw_update() dpy_refresh() • Called on interval to • Called on interval to process guest pixel data update display into DisplaySurface DisplaySurface October 24th, 2013 Citrix Systems, Inc Output Device X11, TCP… 3
- 4. Worst Case Scenario • Emulated VRAM domU dom0 QEMU Virtual VRAM Display Surface SDL Surface (window) ComposiNng Buffer Back Buffer libSDL X.org Linux/DRM Xen October 24th, 2013 Citrix Systems, Inc 4
- 5. ExisNng OpNmizaNons • Foreign Page Mapping (Shared Memory) domU dom0 QEMU Virtual VRAM Dirty Page Data Display Surface SDL Surface (window) ComposiNng Buffer Back Buffer libSDL X.org Linux/DRM Xen October 24th, 2013 Citrix Systems, Inc 5
- 6. ExisNng OpNmizaNons • Shared Buffer Mode + Foreign Page Mapping domU dom0 QEMU Virtual VRAM Display Surface Dirty Page Data SDL Surface (window) ComposiNng Buffer Back Buffer libSDL X.org Linux/DRM Xen October 24th, 2013 Citrix Systems, Inc 6
- 7. Graphics Hardware (The REALLY Short Version) • Systems commonly have a unified memory architecture (UMA) – GPU is connected to the same memory bus as the CPU – Can scan directly out of system RAM • UMA GPUs have their own virtual address space – Can access any domain’s memory if the GPU’s page table is appropriately programmed… – …and… well, it’s the REALLY short version October 24th, 2013 Citrix Systems, Inc 7
- 8. The Obvious SoluNon! • Map the linear framebuffer into the GPU page table • Program the CRTC base address so that the GPU scans out of the framebuffer • Simple! October 24th, 2013 Citrix Systems, Inc 8
- 9. Unpleasant Reality • GPUs are complicated • GPU vendors are secreNve • GPUs change constantly • You probably want to use your GPU for more than one thing at a Nme October 24th, 2013 Citrix Systems, Inc 9
- 10. GEM • Linux already knows how to program your GPU • The API is “standardized” • …but we want to tell it WHICH pages to use. October 24th, 2013 Citrix Systems, Inc 10
- 11. GEM Objects • Basically a bunch of GPU specific state tracking and a scaner-‐gather list of pages • Add an interface that fills in the scaner-‐gather list with the right pages • Pass the object to KMS • … but which pages? October 24th, 2013 Citrix Systems, Inc 11
- 12. Foreign pages? • Mapping the guest’s foreign pages provides virtual addresses to the memory • No machine addresses • Not Linux memory, so no page structs to add to the scaner gather list October 24th, 2013 Citrix Systems, Inc 12
- 13. Grants? • • • • domU allocates a big grant table Creates a grantref for each page in the framebuffer Passes the table address and size to the device model Grant mappings get added to the m2p_override table, so we have page structs • Requires a cooperaNve guest • Uses a lot of grant refs • Sets up mappings that never get used October 24th, 2013 Citrix Systems, Inc 13
- 14. Manual m2p_override • Use the translate_gpfn_list hypercall to get a list of MFNs • Use the m2p_override infrastructure from the grant code to redirect pages • Use redirected pages in the scaner gather list • No guest knowledge • No unnecessary mappings October 24th, 2013 Citrix Systems, Inc 14
- 15. Foreign-‐backed GEM Objects • Add new ioctl to create i915_gem_foreign_object • Standard GEM object, but override get_pages and put_pages handlers using exisNng hooks • Create m2p_override mappings for framebuffer pages • Fill in the scaner-‐gather list with the overridden pages • Pass the object to KMS • Success! October 24th, 2013 Citrix Systems, Inc 15
- 16. New DRM ioctl • (intel) • I915_GEM_FOREIGN – In: • GFN • Size • Domain ID – Out: • GEM handle October 24th, 2013 Citrix Systems, Inc 16
- 17. ExisNng OpNmizaNons • GEM Object + libDRM UI domU dom0 QEMU Virtual VRAM CRTC Scanout Buffer Linux/DRM Display Surface Xen October 24th, 2013 Citrix Systems, Inc 17
- 18. What to do With Your GEM Object • Take over the scanout buffer of an enNre vt – Lowest runNme overhead (none) – Requires source of input events (/dev/event/*) • Convert it to a Prime object for use with DRI2/DRI3 X extensions • Convert it to an EGL named image, and bind it to a texture – Hardware accelerated framebuffer tricks • • • • Scaling RotaNon ComposiNng Effects • Use it to display XenGT hardware accelerated framebuffers – Be sure to see Haitao Shan’s XenGT presentaNon tomorrow… October 24th, 2013 Citrix Systems, Inc 18
- 19. Demo October 24th, 2013 Citrix Systems, Inc 19