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    SubjectRe: [uml-devel] [PATCH] x86, hweight: Fix UML boot crash
    On Sat, Jun 12, 2010 at 16:18, Borislav Petkov <bp@alien8.de> wrote:
    > From: Paolo Giarrusso <p.giarrusso@gmail.com>
    > 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?

    Exactly.

    > 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.

    Regards
    --
    Paolo Giarrusso - Ph.D. Student
    http://www.informatik.uni-marburg.de/~pgiarrusso/


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