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With a vfio assigned device we lay down a base MemoryRegion registered as an IO region, giving us read & write accessors. If the region supports mmap, we lay down a higher priority sub-region MemoryRegion on top of the base layer initialized as a RAM device pointer to the mmap. Finally, if we have any quirks for the device (ie. address ranges that need additional virtualization support), we put another IO sub-region on top of the mmap MemoryRegion. When this is flattened, we now potentially have sub-page mmap MemoryRegions exposed which cannot be directly mapped through KVM. This is as expected, but a subtle detail of this is that we end up with two different access mechanisms through QEMU. If we disable the mmap MemoryRegion, we make use of the IO MemoryRegion and service accesses using pread and pwrite to the vfio device file descriptor. If the mmap MemoryRegion is enabled and results in one of these sub-page gaps, QEMU handles the access as RAM, using memcpy to the mmap. Using either pread/pwrite or the mmap directly should be correct, but using memcpy causes us problems. I expect that not only does memcpy not necessarily honor the original width and alignment in performing a copy, but it potentially also uses processor instructions not intended for MMIO spaces. It turns out that this has been a problem for Realtek NIC assignment, which has such a quirk that creates a sub-page mmap MemoryRegion access. To resolve this, we disable memory_access_is_direct() for ram_device regions since QEMU assumes that it can use memcpy for those regions. Instead we access through MemoryRegionOps, which replaces the memcpy with simple de-references of standard sizes to the host memory. With this patch we attempt to provide unrestricted access to the RAM device, allowing byte through qword access as well as unaligned access. The assumption here is that accesses initiated by the VM are driven by a device specific driver, which knows the device capabilities. If unaligned accesses are not supported by the device, we don't want them to work in a VM by performing multiple aligned accesses to compose the unaligned access. A down-side of this philosophy is that the xp command from the monitor attempts to use the largest available access weidth, unaware of the underlying device. Using memcpy had this same restriction, but at least now an operator can dump individual registers, even if blocks of device memory may result in access widths beyond the capabilities of a given device (RTL NICs only support up to dword). Backports commit 1b16ded6a512809f99c133a97f19026fe612b2de from qemu |
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Unicorn Engine
Unicorn is a lightweight, multi-platform, multi-architecture CPU emulator framework based on QEMU.
Unicorn offers some unparalleled features:
- Multi-architecture: ARM, ARM64 (ARMv8), M68K, MIPS, SPARC, and X86 (16, 32, 64-bit)
- Clean/simple/lightweight/intuitive architecture-neutral API
- Implemented in pure C language, with bindings for Crystal, Clojure, Visual Basic, Perl, Rust, Ruby, Python, Java, .NET, Go, Delphi/Free Pascal and Haskell.
- Native support for Windows & *nix (with Mac OSX, Linux, *BSD & Solaris confirmed)
- High performance via Just-In-Time compilation
- Support for fine-grained instrumentation at various levels
- Thread-safety by design
- Distributed under free software license GPLv2
Further information is available at http://www.unicorn-engine.org
License
This project is released under the GPL license.
Compilation & Docs
See docs/COMPILE.md file for how to compile and install Unicorn.
More documentation is available in docs/README.md.
Contact
Contact us via mailing list, email or twitter for any questions.
Contribute
If you want to contribute, please pick up something from our Github issues.
We also maintain a list of more challenged problems in a TODO list.
CREDITS.TXT records important contributors of our project.