On 2024/07/17 20:03, Phil Dennis-Jordan wrote:
This sequence of patches integrates the paravirtualised graphics device
implemented by macOS's ParavirtualizedGraphics.Framework into Qemu.
Combined with the guest drivers which ship with macOS versions 11 and up,
this allows the guest OS to use the host's GPU for hardware accelerated
3D graphics, GPGPU compute (both using the 'Metal' graphics API), and
window compositing.
Some background:
----------------
The device exposed by the ParavirtualizedGraphics.Framework's (henceforth
PVG) public API consists of a PCI device with a single memory-mapped BAR;
the VMM is expected to pass reads and writes through to the framework, and
to forward interrupts emenating from it to the guest VM.
The bulk of data exchange between host and guest occurs via shared memory,
however. For this purpose, PVG makes callbacks to VMM code for allocating,
mapping, unmapping, and deallocating "task" memory ranges. Each task
represents a contiguous host virtual address range, and PVG expects the
VMM to map specific guest system memory ranges to these host addresses via
subsequent map callbacks. Multiple tasks can exist at a time, each with
many mappings.
Data is exchanged via an undocumented, Apple-proprietary protocol. The
PVG API only acts as a facilitator for establishing the communication
mechanism. This is perhaps not ideal, and among other things means it
only works on macOS hosts, but it's the only serious option we've got for
good performance and quality graphics with macOS guests at this time.
The first iterations of this PVG integration into Qemu were developed
by Alexander Graf as part of his "vmapple" machine patch series for
supporting aarch64 macOS guests, and posted to qemu-devel in June and
August 2023:
https://lore.kernel.org/all/20230830161425.91946-1-g...@amazon.com/T/
This integration mimics the "vmapple"/"apple-gfx" variant of the PVG device
used by Apple's own VMM, Virtualization.framework. This variant does not use
PCI but acts as a direct MMIO system device; there are two MMIO ranges, one
behaving identically to the PCI BAR, while the other's functionality is
exposed by private APIs in the PVG framework. It is only available on aarch64
macOS hosts.
I had prior to this simultaneously and independently developed my own PVG
integration for Qemu using the public PCI device APIs, with x86-64 and
corresponding macOS guests and hosts as the target. After some months of
use in production, I was slowly reviewing the code and readying it for
upstreaming around the time Alexander posted his vmapple patches.
I ended up reviewing the vmapple PVG code in detail; I identified a number
of issues with it (mainly thanks to my prior trial-and-error working with
the framework) but overall I thought it a better basis for refinement
than my own version:
- It implemented the vmapple variant of the device. I thought it better to
port the part I understood well (PCI variant) to this than trying to port
the part I didn't understand well (MMIO vmapple variant) to my own code.
- The code was already tidier than my own.
It also became clear in out-of-band communication that Alexander would
probably not end up having the time to see the patch through to inclusion,
and was happy for me to start making changes and to integrate my PCI code.
I think you also need to take over the base vmapple change because PVG
cannot be tested without it; Only macOS can use PVG, and macOS requires
vmapple in my understanding.
Regards,
Akihiko Odaki
