On Jul 31, 2017, at 5:23 PM, Bae, Chang Seok <chang.seok....@intel.com> wrote:
>> 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 That won't be reliable -- LDTR could change more than once and be reused between context switches. If we went this route, I think we'd put a u64 version in ldt_struct. We'd also need to audit and fix up every access to thread.fs/gsbase.
