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

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Message-ID: <9D8B895D-0728-4451-BD22-B8EC78F90BEB@zytor.com>
Date:   Fri, 23 Dec 2022 11:37:07 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Xin Li <xin3.li@...el.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>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "ravi.v.shankar@...el.com" <ravi.v.shankar@...el.com>
Subject: Re: [RFC PATCH 22/32] x86/fred: FRED initialization code

On December 20, 2022 1:55:31 AM PST, Andrew Cooper <Andrew.Cooper3@...rix.com> 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... */
>>> +	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
>

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