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Date:   Fri, 23 Dec 2022 11:37:07 -0800
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Andrew Cooper <Andrew.Cooper3@citrix.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Xin Li <xin3.li@intel.com>
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        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
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        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        "seanjc@google.com" <seanjc@google.com>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "ravi.v.shankar@intel.com" <ravi.v.shankar@intel.com>
Subject: Re: [RFC PATCH 22/32] x86/fred: FRED initialization code
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On December 20, 2022 1:55:31 AM PST, Andrew Cooper <Andrew=2ECooper3@citrix=
=2Ecom> 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=2E=2E=2E */
>>> +	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=2E=2E IIRC fred only switches to another stack when the level=
 of
>> the exception is higher=2E Specifically, if we trigger #DB while inside
>> #NMI we will not switch to the #DB stack (since 1 < 2)=2E
>
>There needs to be a new stack for #DF, and just possibly one for #MC=2E=
=C2=A0
>NMI and #DB do not need separate stacks under FRED=2E
>
>> 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=2E=C2=A0 #DB shouldn't get too deep, and NMI gets properly
>inhibited now=2E
>
>~Andrew
>

I still don't think you want to take #DB or =E2=80=93 especially =E2=80=93=
 NMI on the task stack while in the kernel=2E In fact, the plan is to get r=
id of the software irqstack handling, too, but at tglx's request that will =
be a later changeset (correctness first, then optimization=2E)