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SubjectRE: [RFC PATCH 22/32] x86/fred: FRED initialization code
On December 20, 2022 9:28:52 PM PST, "Li, Xin3" <xin3.li@intel.com> 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.

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