Re: [uml-devel] [PATCH] x86, hweight: Fix UML boot crash

From: Paolo Giarrusso
Date: Sat Jun 12 2010 - 12:02:17 EST

On Sat, Jun 12, 2010 at 16:18, Borislav Petkov <bp@xxxxxxxxx> wrote:
> From: Paolo Giarrusso <p.giarrusso@xxxxxxxxx>
> Date: Sat, Jun 12, 2010 at 03:34:38PM +0200
> Hi,
>> > That looks better to me, although I'm still wondering why UML can't
>> > stomach the register-saving tricks... it is not at all "obvious" why
>> > that can't be done.
>> Hi all, and sorry for the delay, I hope you still care about this.
>> First, ARCH_HWEIGHT_CFLAGS should IMHO be shared with UML. I.e., moved
>> to arch/x86/Kconfig.cpu (which was born as Kconfig code shared with
>> UML), or copied in UML (it's not defined, as far as I can see).
>> Otherwise it just can't work. And I think that's it.

Just to be sure: by "that's it" I meant "this is the problem".
You didn't answer here - did you see it? What do you think? Can you
try the one-line fix at some point?
Just to make it clear: I've not been actively developing UML (or
almost anything in kernel space) for ages (~4 years), so it's unlikely
that I'll try fixing this. It just happens that things on the UML
front stayed mostly the same, so I thought that my knowledge of the
code is still useful.

>> Second, I've been looking at arch_hweight.h to try answering as well,
>> and my question is: did somebody ever implement ALTERNATIVE support on
>> UML? When I worked on it, this thing didn't exist at all. The user
>> declared the host CPU, and we enabled features based on that. There's
>> barely code for exception tables, and we never used it to implement
>> copy_from_user and staff like that (I recall the exception handler was
>> set at run-time).

>> Indeed, arch/um/kernel/um_arch.c:apply_alternatives() is empty. And I
>> mean, implementing it is not so trivial (unlike exception handling),
>> simply because it requires making the binary mapping writable, and I'm
>> not sure UML does it already.

> Which would mean that UML doesn't use alternatives at all and uses the
> instructions which are meant to be replaced instead, no?


> In that case,
> fixing this is either by rerouting the includes (easiest, already in
> -tip) or adding alternatives support (harder, needs volunteers :)).

Well, even doing just nothing should work, if you fix the trivial
thing above (which at least for 64bit should work).

>> A third note is that UML links with glibc, so it can have a different
>> calling convention from the kernel. Say, on x86 32bit regparm doesn't
>> work (in fact, -mregparm is set in arch/x86/Makefile and not in
>> arch/x86/Makefile_32.cpu). And since popcnt is supported on 32bit, it
>> might in theory make a difference for that case. But maybe those flags
>> are simply fine, I didn't recheck the possible calling conventions.

> If this is also the case, the -fcall-saved-* stuff won't work on UML and
> yet another way of doing "call *func" from within asm("...") and making
> sure the callee doesn't clobber caller's regs will be needed for UML.

Hmpf... anyway, 64bit should be fine since there's just one calling
convention, everywhere, and already regparm'ed.

Paolo Giarrusso - Ph.D. Student
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