On Mar 28, 2015 1:35 AM, "Ingo Molnar" <mingo@xxxxxxxxxx> wrote: > > > * Andy Lutomirski <luto@xxxxxxxxxx> wrote: > > > Sysexit is scary on 64-bit kernels -- sysexit must be invoked with > > usergs and IRQs on. That means that we rely on sti to correctly > > mask interrupts for one instruction. This is okay by itself, but > > the semantics with respect to NMIs are unclear. > > At least judging by profiling output I think NMIs observe the STI > window of one instruction non-execution as well. (But I'm not 100% > sure.) > > > Avoid the whole issue by using sysretl instead. For background, > > Intel CPUs don't allow syscall from compat mode, but they do allow > > sysret back to compat mode. Go figure. > > > > Oddly this seems to be 30 cycles or so faster. Avoiding popfq and > > sti will account for under half of that, I think, so my best guess > > is that Intel just optimizes sysret much better than sysexit. > > > > Cc: stable@xxxxxxxxxxxxxxx > > I like it, but no way is this automatic -stable material ... if proven > upstream we can forward it as a fix for SYSEXIT fragility, but not > automatically, IMHO. Agreed. I wish we had a Stable-after-a-long-soak tag. --Andy -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html