Messages in this thread | | | From | "Bae, Chang Seok" <> | Subject | RE: FSGSBASE ABI considerations | Date | Mon, 31 Jul 2017 21:23:45 +0000 |
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> On an FSGSBASE-enabled system, I think we need to provide deterministic, documented, tested behavior. I can think of three plausible choices: > 1a. modify_ldt() immediately updates FSBASE and GSBASE all threads that reference the modified selector. > 1b. modify_ldt() immediatley updates FSBASE and GSBASE on all threads that reference the LDT. > 2. modify_ldt() leaves FSBASE and GSBASE alone on all threads. > (2) is trivial to implement, whereas (1a) and (1b) are a bit nasty to implement when FSGSBASE is on.
> The tricky bit is that 32-bit kernels can't do (2), so, if we want modify_ldt() to behave the same on 32-bit and 64-bit kernels, we're stuck with (1).
While implementing (1) is still unclear for context switch, here is one idea for (1b): - thread struct has new entry for ldt pointer that last seen - modify_ldt happens - ldtr upated for active threads via IPI - for inactive threads being scheduled in, ldtr updated before __switch_to - in __switch_to, read ldtr by sldt and compare the new ldt pointer sldt is ucode that likely takes only a couple cycles - mostly matched given modify_ldt is rare - unmatched, don't write gsbase if gs indicating LDT
> (I think we can implement (2) with acceptable performance on 64-bit non-FSGSBASE kernels if we wanted to.) Nonetheless, with Andy's argument for (1), (2) might be straightforward assuming that user code already followed the legacy around modify_ldt in 64bit.
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