Hi Philippe,
> 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.
Yes, so the sixth patch ("[PATCH 06/10] include/exec: Allow using 64bit
guest addresses on emscripten") should be considered as a temporary
workaround, enabled only for Emsripten builds. It will be removed once
wasm64 gains broader support and is adopted in the Wasm backend.
