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[Qemu-devel] linux-user: drop support for "unknown" host CPUs (ie hppa a

From: Peter Maydell
Subject: [Qemu-devel] linux-user: drop support for "unknown" host CPUs (ie hppa and m68k) ?
Date: Mon, 13 Jun 2016 10:26:27 +0100

Currently our configure script allows linux-user targets to be built
for "unknown" host CPU architectures (which must be using the TCI
interpreter). However, code like user-exec.c has host-architecture
#ifdef ladders which in practice mean you can't build linux-user for
an arbitrary host architecture. The only two host architectures which
have code in user-exec.c but aren't recognised by configure are
HPPA and m68k.

I'd like to move to an "every supported host architecture has a
linux-user/host/$(ARCH)/hostdep.h header" model (this fixes a problem
in the code currently in master where make doesn't notice it needs to
rebuild if a host arch moves from "using generic/" to "using $(ARCH)").
Host architectures which we sort-of-but-don't support are an annoying
corner case I'd like to be able to drop entirely, by either:
 (1) just dropping the support outright
 (2) promoting them to at least being recognised by configure as
   a known architecture, even if one without a tcg backend

Does this seem like a good idea?

I think HPPA should definitely be in category (1) -- we dropped
the TCG backend years back, and I don't think any of us has a
machine to test changes on,.

Which category should m68k host support go into? Does anybody
actually use m68k host + TCI interpreter linux-user ? My guess
is this isn't actually used, because the m68k-specific code in
user-exec.c doesn't set the "is_write" flag correctly, which means
that guests which do self-modifying code won't work (including
anything using a signal handler with an on-stack trampoline).
So my preference would be to drop the m68k-host code too.

-- PMM

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