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Date:   Tue, 20 Dec 2022 10:45:19 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Xin Li <xin3.li@intel.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org, kvm@vger.kernel.org,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, hpa@zytor.com,
        andrew.cooper3@citrix.com, seanjc@google.com, pbonzini@redhat.com,
        ravi.v.shankar@intel.com
Subject: Re: [RFC PATCH 22/32] x86/fred: FRED initialization code
Message-ID: <Y6GELyEJeKY3dEqJ@hirez.programming.kicks-ass.net>
References: <20221220063658.19271-1-xin3.li@intel.com>
 <20221220063658.19271-23-xin3.li@intel.com>
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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?