> > > Trying to understand more what prevents SSS to enable in pre FRED, Is > > > it better #CP exception > > > handling with other nested exceptions? > > > > SSS took the syscall gap and made it worse -- as in *way* worse. > > > > To top it off, the whole SSS busy bit thing is fundamentally > > incompatible with how we manage to survive nested exceptions in NMI > > context. > > > > Basically, the whole x86 exception / stack switching logic was already > > borderline impossible (consider taking an MCE in the early NMI path > > where we set up, but have not finished, the re-entrancy stuff), and > > SSS > > > pushed it over the edge and set it on fire. ah I see. SSS takes it to the next level. > > > > And NMI isn't the only problem, the various new virt exceptions #VC and > > #HV are on their own already near impossible, adding SSS again pushes > > the whole thing into clear insanity. > > > > There's a good exposition of the whole trainwreck by Andrew here: > > > > https://www.youtube.com/watch?v=qcORS8CN0ow > > > > (that is, sorry for the youtube link, but Google is failing me in > > finding the actual Google Doc that talk is based on, or even the slide > > deck :/) I think I got the link: https://docs.google.com/document/d/1hWejnyDkjRRAW-JEsRjA5c9CKLOPc6VKJQsuvODlQEI/edit?pli=1 > > > > > > > > FRED solves all that by: > > > > - removing the stack gap, cc/ip/ss/sp/ssp/gs will all be switched > > atomically and consistently for every transition. > > > > - removing the non-reentrant IST mechanism and replacing it with stack > > levels > > > > - adding an explicit NMI latch > > > > - re-organising the actual shadow stacks and doing away with that busy > > bit thing (I need to re-read the FRED spec on this detail again). > > Thank you for explaining. I will also study the FRED spec and corresponding kernel patches posted in the mailing list. > > > > > > Crazy as we are, we're not touching legacy/IDT SSS with a ten foot pole, > > sorry. ya, interesting. Best regards, Pankaj
