On Fri, 22 Jan 2021 at 08:50, Max Reitz <mre...@redhat.com> wrote: > > On 20.01.21 18:25, Laszlo Ersek wrote: > > [...] > > > A simple grep for SIGUSR2 seems to indicate that SIGUSR2 is not used by > > system emulation for anything else, in practice. Is it possible to > > dedicate SIGUSR2 explicitly to coroutine-sigaltstack, and set up the > > action beforehand, from some init function that executes on a "central" > > thread, before qemu_coroutine_new() is ever called? > > I wrote a patch to that effect, but just before sending I wondered > whether SIGUSR2 cannot be registered by the “guest” in user-mode > emulation, and whether that would then break coroutines from there on. > > (I have no experience dealing with user-mode emulation, but it does look > like the guest can just register handlers for any signal but SIGSEGV and > SIGBUS.)
Yes, SIGUSR2 is for the guest in user-emulation mode. OTOH do we even use the coroutine code in user-emulation mode? Looking at the meson.build files, we only add the coroutine_*.c to util_ss if 'have_block', and we set have_block = have_system or have_tools. I think (but have not checked) that that means we will build and link the object file into the user-mode binaries if you happen to build them in the same run as system-mode binaries, but won't build them in if you built the user-mode binaries as a separate build. Which is odd and probably worth fixing, but does mean we know that we aren't actually using coroutines in user-mode. (Also user-mode really means Linux or BSD and I think both of those have working ucontext.) thanks -- PMM
