Re: [Qemu-devel] patch to swap SIGRTMIN + 1 and SIGRTMAX - 1

[Aleksandar Markovic]

On Wednesday, August 28, 2019, Aleksandar Markovic <address@hidden> wrote:
> > From: Laurent Vivier <address@hidden>
> > Sent: Wednesday, August 28, 2019 10:51 AM
> > To: Josh Kunz; Aleksandar Markovic; address@hidden
> > Cc: address@hidden; address@hidden; address@hidden; > address@hidden; Peter Maydell; Shu-Chun Weng; Aleksandar Markovic
> > Subject: [EXTERNAL]Re: [Qemu-devel] patch to swap SIGRTMIN + 1 and SIGRTMAX - 1
> > 
> > Le 26/08/2019 à 23:10, Josh Kunz a écrit :
> > > On Wed, Aug 21, 2019 at 2:28 AM Laurent Vivier <address@hidden
> > > <mailto:address@hidden>> wrote:
> > >
> > >     Le 19/08/2019 à 23:46, Josh Kunz via Qemu-devel a écrit :
> > >     > Hi all,
> > >     >
> > >     > I have also experienced issues with SIGRTMIN + 1, and am interested in
> > >     > moving this patch forwards. Anything I can do here to help? Would the
> > >     > maintainers prefer myself or Marli re-submit the patch?
> > >     >
> > >     > The Go issue here seems particularly sticky. Even if we update the Go
> > >     > runtime, users may try and run older binaries built with older
> > >     versions of
> > >     > Go for quite some time (months? years?). Would it be better to
> > >     hide this
> > >     > behind some kind of build-time flag
> > >     (`--enable-sigrtmin-plus-one-proxy` or
> > >     > something), so that some users can opt-in, but older binaries
> > >     still work as
> > >     > expected?
> > >     >
> > >     > Also, here is a link to the original thread this message is in
> > >     reply to
> > >     > in-case my mail-client doesn't set up the reply properly:
> > >     > https://lists.nongnu.org/archive/html/qemu-devel/2019-07/msg01303.html
> > >
> > >     The problem here is we break something to fix something else.
> > >
> > >     I'm wondering if the series from Aleksandar Markovic, "linux-user:
> > >     Support signal passing for targets having more signals than host" [1]
> > >     can fix the problem in a better way?
> > >
> > >
> > > That patch[1] (which I'll refer to as the MUX patch to avoid confusion)
> > > does not directly fix the issue addressed by this patch (re-wiring
> > > SIGRTMIN+1), but since it basically implements generic signal
> > > multiplexing, it could be re-worked to address this case as well. The
> > > way it handles `si_code` spooks me a little bit. It could easily be
> > > broken by a kernel version change, and such a breakage could be hard to
> > > detect or lead to surprising results. Other than that, it looks like a
> > > reasonable implementation.
> > >
> > > That said, overall, fixing the SIGRTMIN+1 issue using a more-generic
> > > signal-multiplexing mechanism doesn't seem *that* much better to me. It
> > > adds a lot of complexity, and only saves a single signal (assuming glibc
> > > doesn't add more reserved signals). The "big win" is additional
> > > emulation features, like those introduced in MUX patch (being able to
> > > utilize signals outside of the host range). If having those features in
> > > QEMU warrants the additional complexity, then re-working this patch
> > > on-top of that infrastructure seems like a good idea.
> > >
> > > If the maintainers want to go down that route, then I would be happy to
> > > re-work this patch utilizing the infrastructure from the MUX patch.
> > > Unfortunately it will require non-trivial changes, so it may be best to
> > > wait until that patch is merged. I could also provide a patch "on top
> > > of" the MUX patch, if that's desired/more convenient.
> > >
> > > Just one last note, if you do decide to merge the MUX patch, then it
> > > would be best to use SIGRTMAX (instead of SIGRTMAX-1) as the
> > > multiplexing signal if possible, to avoid breaking go binaries.
> > >
> > 
> > Personally, I prefer a solution that breaks nothing.
> > 
> > Aleksandar, Milos,
> > 
> > do you have an updated version of you series "Support signal passing for
> > targets having more signals than host"?
> > 
>
> Milos is unfortunetely working on an entirely different project now, and can't spare enough time to finish the series. I am also busy with other issues, even though I would like very much this or equivalent solution to be integrated. Alternatively, someone in the team may have time later this year, but I do not know that yet  - perhaps somebody else (Josh) can take over the series?

Hello, all.

From my side, status quo. Milos (who ironically works in the office next to mine) expressed interest in modifying his solution to be in accordance with what Peter said, says that this is doable, but requires a lot of meticulous work - however, he is too involved into his project for months to go. I am also involved in other things, and, furthermore, has less background knowledge than Milos. That said, we in MIPS would like the solution described by Peter as much as other platforms, if not more.

All this taken into account, perhaps this can be a project for Outreachy or Google Summer of Code, mentored by Peter?

The perhaps largest problem with that is that the person doing it would need some steep learning curve (on signals in general, and their treatment in QEMU), that can take some significant time.

Yours,

Aleksandar

 

> Sincerely,
> Aleksandar
>
>
> > Thanks,
> > Laurent
> >