On Thu, Aug 13, 2020 at 02:09:33PM -0400, Arvind Sankar wrote: > 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 <ndesaulni...@google.com> writes: > > > > On Thu, Aug 6, 2020 at 3:11 PM Thomas Gleixner <t...@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/4755a809.4050...@qumranet.com/ > > > > Patch 4: > > > > https://lore.kernel.org/lkml/11967844071346-git-send-email-gco...@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
Whew!!! ;-) Thanx, Paul