Messages in this thread | | | From | Arvind Sankar <> | Date | Thu, 13 Aug 2020 14:09:33 -0400 | Subject | Re: [PATCH] x86: work around clang IAS bug referencing __force_order |
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On Thu, Aug 13, 2020 at 10:37:01AM -0700, Paul E. McKenney wrote: > On Thu, Aug 13, 2020 at 07:28:57PM +0200, Thomas Gleixner wrote: > > Nick Desaulniers <ndesaulniers@google.com> writes: > > > On Thu, Aug 6, 2020 at 3:11 PM Thomas Gleixner <tglx@linutronix.de> wrote: > > >> > + * > > >> > + * Clang sometimes fails to kill the reference to the dummy variable, so > > >> > + * provide an actual copy. > > >> > > >> Can that compiler be fixed instead? > > > > > > I don't think so. The logic in the compiler whether to emit an > > > > Forget that I asked. Heat induced brain damaged. > > > > > I'd much rather remove all of __force_order. > > > > Right. > > > > > Not sure about the comment in arch/x86/include/asm/special_insns.h > > > either; smells fishy like a bug with a compiler from a long time ago. > > > It looks like it was introduced in: > > > commit d3ca901f94b32 ("x86: unify paravirt parts of system.h") > > > Lore has this thread: > > > https://lore.kernel.org/lkml/4755A809.4050305@qumranet.com/ > > > Patch 4: https://lore.kernel.org/lkml/11967844071346-git-send-email-gcosta@redhat.com/ > > > It seems like there was a discussion about %cr8, but no one asked > > > "what's going on here with __force_order, is that right?" > > > > Correct and the changelog is uselss in this regard. > > > > > Quick boot test of the below works for me, though I should probably > > > test hosting a virtualized guest since d3ca901f94b32 refers to > > > paravirt. Thoughts? > > > > Let me ask (hopefully) useful questions this time: > > > > Is a compiler allowed to reorder two 'asm volatile()'? > > > > Are there compilers (gcc >= 4.9 or other supported ones) which do that? > > I would hope that the answer to both of these questions is "no"! > > But I freely confess that I have been disappointed before on this sort > of thing. :-/ > > Thanx, Paul
Ok, I found this, so gcc developers consider re-ordering volatile asm wrt each other a bug at least.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602
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