Messages in this thread | | | Date | Thu, 13 Aug 2020 10:37:01 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] x86: work around clang IAS bug referencing __force_order |
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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
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