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

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Date:   Tue, 20 Dec 2022 10:45:19 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Xin Li <xin3.li@...el.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com,
        andrew.cooper3@...rix.com, seanjc@...gle.com, pbonzini@...hat.com,
        ravi.v.shankar@...el.com
Subject: Re: [RFC PATCH 22/32] x86/fred: FRED initialization code

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).

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?