On Mon, Mar 21, 2016 at 3:41 PM, Andi Kleen <[email protected]> wrote: >> Imagine that some brilliant lightweight threading library does: >> >> - set GS to nonzero (by whatever means -- arch_prctl(ARCH_SET_GS, >> whatever) on a pre-IVB host followed by migration, some modify_ldt >> garbage, simple bloody-mindedness, whatever); > > Migration is only possible when the CPUID flags match. > >> - WRGSBASE >> - Use GS for a bit >> >> This will work most of the time until it gets unlucky with preemption. > > As soon as a kernel thread or something else schedules the value > will be lost. > >> And yes, runtime library authors really do mess up in amazing ways. >> >> It's an issue. It needs conscious design. > > Ok. So your only objection is the order of the context switch > updates?
No. My objection is that there needs to be an explicit statement what the semantics are. If the agreed-upon semantics are "undefined behavior if GS != 0 and GSBASE doesn't match the descriptor", so be it, but this needs to be a conscious decision and needs to be weighed against the alternatives. The actual implementation details are just details. They need to match the intended semantics, of course. --Andy
