linux-kernel - RE: [RFC PATCH 22/32] x86/fred: FRED initialization code

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Date:   Tue, 20 Dec 2022 21:44:33 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     "Li, Xin3" <xin3.li@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: RE: [RFC PATCH 22/32] x86/fred: FRED initialization code

On December 20, 2022 9:28:52 PM PST, "Li, Xin3" <xin3.li@...el.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.