> > >> + wrmsrl(MSR_IA32_FRED_STKLVLS, > > >> + FRED_STKLVL(X86_TRAP_DB, 1) | > > >> + FRED_STKLVL(X86_TRAP_NMI, 2) | > > >> + FRED_STKLVL(X86_TRAP_MC, 2) | > > >> + FRED_STKLVL(X86_TRAP_DF, 3)); > > >> + > > >> + /* The FRED equivalents to IST stacks... */ > > >> + wrmsrl(MSR_IA32_FRED_RSP1, __this_cpu_ist_top_va(DB)); > > >> + wrmsrl(MSR_IA32_FRED_RSP2, __this_cpu_ist_top_va(NMI)); > > >> + wrmsrl(MSR_IA32_FRED_RSP3, __this_cpu_ist_top_va(DF)); > > > Not quite.. IIRC fred only switches to another stack when the level > > > of the exception is higher. Specifically, if we trigger #DB while > > > inside #NMI we will not switch to the #DB stack (since 1 < 2). Yes, current stack level can only grow higher. > > > > There needs to be a new stack for #DF, and just possibly one for #MC. > > NMI and #DB do not need separate stacks under FRED. > > True, there is very little need to use additional stacks with FRED. Pretty much. #DB/NMI from a ring 3 context uses CSL 0, and their CSLs increase only when happening from a ring 0 context. > > > > Now, as mentioned elsewhere, it all nests a lot saner, but stack > > > exhaustion is still a thing, given the above, what happens when a > > > #DB hits an #NMI which tickles a #VE or something? > > > > > > I don't think we've increased the exception stack size, but perhaps > > > we should for FRED? > > > > Not sure if it matters too much - it doesn't seem usefully different > > to IDT delivery. #DB shouldn't get too deep, and NMI gets properly > > inhibited now. > > Both #DB and #NMI can end up in perf, and all that goes quite deep :/ Can you please elaborate it a bit?
