On Wed, 26 Jul 2017 08:05:27 +0200 Heiko Carstens <heiko.carstens@xxxxxxxxxx> wrote: > On Wed, Jul 26, 2017 at 07:40:44AM +0200, Martin Schwidefsky wrote: > > On Wed, 26 Jul 2017 07:00:33 +0200 > > Heiko Carstens <heiko.carstens@xxxxxxxxxx> wrote: > > > > > On Tue, Jul 25, 2017 at 06:14:51PM -0700, Guenter Roeck wrote: > > > > Hi Martin, > > > > > > > > my s390 qemu tests in linux-next stopped working a few days ago. > > > > Bisect points to commit 's390/spinlock: add niai spinlock hints'. > > > > > > > > Looking at the patch, this isn't really surprising; at least to me it looks > > > > like the patch is making instructions mandatory which are only available in > > > > Z14 CPUs. Does this mean that older s390 CPUs (such as the Z900 used in my > > > > qemu tests) are no longer going to be supported in Linux ? > > > > > > No, that means that the patch has a bug. The NIAI instruction is only > > > available if the execution-hint facility is installed. That facility came > > > with zEC12. Luckily it uses the same facility indicator bit like the > > > miscellaneous-instruction-extensions facility, which we already use anyway > > > if the kernel gets compiled for zEC12. In that case we have early code > > > which verifies if all required facilities to run the kernel are installed, > > > and if not it will print a message to the console and stop the machine. > > > > > > So the easiest fix would be to generate the NIAI instruction only if the > > > kernel gets compiled for zEC12 or newer. > > > > Hmm, I though that NIAI is a NOP on older machines. A runtime check for > > the facility bit is out of the question as the NIAI-7 gets inlined in > > the spin_unlock code. So yes, the only available fix is to make the > > NIAI hinting conditional on zEC12. Which is quite ugly as we would need > > an architecture level set to zEC12 for the distribution kernel to make > > use of NIAI. > > Alternatively you could generate a four-byte nop, and replace that at IPL > time with the needed NIAI instruction, if the facility is available. Some > sort of "alternative" code patching infrastructure that x86 already has. > Not sure if it is worth it, however... Patching all spin_unlock inlines? There are a lot of callers for this function. We could think about an out-of-line spin_unlock and patch this single function but then we'd loose the advantage of inlining. I do not think it is worthwhile. I pushed an updated patch to the features branch of s390/linux. Should be in linux-next tomorroy. Thanks. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin. -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
