On 9/4/25 21:21, Stefan Hajnoczi wrote:
On Mon, Apr 07, 2025 at 11:45:51PM +0900, Kohei Tokunaga wrote:
This patch series enables QEMU's system emulator to run in a browser using
Emscripten.
It includes implementations and workarounds to address browser environment
limitations, as shown in the following.
I think it would be great to merge this even if there are limitations
once code review comments have been addressed. Developing WebAssembly
support in-tree is likely to allow this effort to develop further than
if done in personal repos (and with significant efforts required to
rebase the code periodically).
# New TCG Backend for Browsers
A new TCG backend translates IR instructions into Wasm instructions and runs
them using the browser's WebAssembly APIs (WebAssembly.Module and
WebAssembly.instantiate). To minimize compilation overhead and avoid hitting
the browser's limitation of the number of instances, this backend integrates
a forked TCI. TBs run on TCI by default, with frequently executed TBs
compiled into WebAssembly.
# Workaround for Running 64-bit Guests
The current implementation uses Wasm's 32-bit memory model, even though Wasm
supports 64-bit variables and instructions. This patch explores implementing
TCG 64-bit instructions while leveraging SoftMMU for address translation. To
enable 64-bit guest support in Wasm today, it was necessary to partially
revert recent changes that removed support for different pointer widths
between the host and guest (e.g., commits
a70af12addd9060fdf8f3dbd42b42e3072c3914f and
bf455ec50b6fea15b4d2493059365bf94c706273) when compiling with
Emscripten. While this serves as a temporary workaround, a long-term
solution could involve adopting Wasm's 64-bit memory model once it gains
broader support, as it is currently not widely adopted (e.g., unsupported by
Safari and libffi). Feedback and suggestions on this approach are welcome.
The biggest problem I'm seeing is we no longer support 64-bit guests on
32-bit hosts, and don't plan to revert that.
