On Tue, Dec 20, 2022 at 09:55:31AM +0000, Andrew Cooper wrote: > On 20/12/2022 9:45 am, Peter Zijlstra wrote: > > On Mon, Dec 19, 2022 at 10:36:48PM -0800, Xin Li 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). > > 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. > > 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 :/
