Re: [PATCH 00/10] Enable QEMU to run on browsers

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

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.