Jes, You mentioned that you're running on KVM... while I don't know much about how KVM works, is that load instruction being intercepted and emulated by any chance? (Considering that it looks like something that would be accessing hardware registers on a real machine.) A bug in that emulation could explain why it's behaving strangely... Matt On Mon, May 19, 2008 at 05:27:26AM -0400, Jes Sorensen wrote: > Hi, > > Trying to figure out if it's GCC being broken or what else could be > causing this problem. > > I am chasing an issue where the console hangs when I boot my self > compiled kernels under KVM. I have tracked down the problem to a > single instruction change in the vgacon driver, however if I > understand the rules correctly this specific change should not cause > any difference in code execution whatsoever? > > Basically if I apply the below diff my code no longer works. However I > am under the impression that ld2.acq does sign extension? If thats the > case, does anybody have an explanation for why my bode breaks if I > don't perform the zxt2 instruction on the previously loaded register > before the comparison? Note the mov r19=r17 at the end, which should > mean there is no reason why this change should affect code flow > further down the lin... > > Any input would be greatly appreciated, I have been trying to track > this down for a couple of weeks and I still don't get why this is > going wrong. > > Thanks, > Jes > > > --- /tmp/vgacon-good.S 2008-05-16 06:09:08.000000000 -0500 > +++ /tmp/vgacon-test2.S 2008-05-19 03:54:45.000000000 -0500 > @@ -113,22 +113,22 @@ > ld2.acq r21 = [r18], 2 > ;; > ld2.acq r16 = [r18] > nop 0 > .mmi > st2.rel [r8] = r20 > st2.rel [r18] = r22 > zxt2 r15 = r21 > .mii > - ld2.acq r19 = [r8] > + ld2.acq r17 = [r8] > nop 0 > ;; > - zxt2 r17 = r19 > + zxt2 r19 = r19 > .mib > nop 0 > zxt2 r16 = r16 > nop 0 > ;; > .mib > cmp4.ne p8, p9 = r11, r17 > mov r19 = r17 > (p8) br.cond.dpnt .L8 > -- > To unsubscribe from this list: send the line "unsubscribe linux-ia64" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe kvm-ia64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
