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Date: Sat, 12 Jun 2010 18:01:44 +0200 From: Paolo Giarrusso <p.giarrusso@...il.com> To: Borislav Petkov <bp@...en8.de>, Paolo Giarrusso <p.giarrusso@...il.com>, "H. Peter Anvin" <hpa@...or.com>, Geert Uytterhoeven <geert@...ux-m68k.org>, Borislav Petkov <borislav.petkov@....com>, Toralf Förster <toralf.foerster@....de>, user-mode-linux-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org, Jeff Dike <jdike@...toit.com> Subject: Re: [uml-devel] [PATCH] x86, hweight: Fix UML boot crash On Sat, Jun 12, 2010 at 16:18, Borislav Petkov <bp@...en8.de> wrote: > From: Paolo Giarrusso <p.giarrusso@...il.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/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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