On December 20, 2022 9:28:52 PM PST, "Li, Xin3" <xin3.li@xxxxxxxxx> wrote: >> > >> + 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? > Right, this is one major reason for putting #DB/NMI in a separate stack level.
