On 07/19/17 19:21, H. Peter Anvin wrote: > On 07/19/17 16:33, H. Peter Anvin wrote: >>> >>> I agree that it is odd but that's how the compiler generates code. I >>> will re-explore PIC options with mcmodel=small or medium, as mentioned >>> on other threads. >> >> Why should the way compiler generates code affect the way we do things >> in assembly? >> >> That being said, the compiler now has support for generating this kind >> of code explicitly via the __seg_gs pointer modifier. That should let >> us drop the __percpu_prefix and just use variables directly. I suspect >> we want to declare percpu variables as "volatile __seg_gs" to account >> for the possibility of CPU switches. >> >> Older compilers won't be able to work with this, of course, but I think >> that it is acceptable for those older compilers to not be able to >> support PIE. >> > > Grump. It turns out that the compiler doesn't do the right thing for > symbols marked with the __seg_[fg]s markers. __thread does the right > thing, but __thread a) has %fs: hard-coded, still, and b) I believe can > still cache %seg:0 arbitrarily long.
I filed this bug report for gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81490 It might still be possible to work around this by playing really ugly games with __thread, but I haven't yet figured out how best to do that. -hpa _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org https://lists.xen.org/xen-devel
