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. > 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. ~Andrew
