Re: [RFC PATCH 22/32] x86/fred: FRED initialization code

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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




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