linux-kernel - Re: [PATCH v1 1/1] x86/fred: Fix the FRED RSP0 MSR out of sync with its per CPU cache

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Message-ID: <a6bc203b-b1e2-4da1-b1f4-1a61bb35d931@citrix.com>
Date: Wed, 8 Jan 2025 21:10:06 +0000
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Dave Hansen <dave.hansen@...el.com>, "Xin Li (Intel)" <xin@...or.com>,
 linux-kernel@...r.kernel.org
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com
Subject: Re: [PATCH v1 1/1] x86/fred: Fix the FRED RSP0 MSR out of sync with
 its per CPU cache

On 08/01/2025 8:14 pm, Dave Hansen wrote:
> On 1/7/25 18:36, Xin Li (Intel) wrote:
>> +	 * Another option is to leave the FRED RSP0 MSR as-is, because the RESET
>> +	 * state of FRED MSRs is zero and INIT does not change the value of the
>> +	 * FRED MSRs in a CPU offline/online cycle.  But it doesn't seem safe to
>> +	 * depend on the properties of INIT as that's way too many things that
>> +	 * could cause bugs.
> Wouldn't that also open us up to kexec problems? If the last kernel
> (maybe not even Linux) left RSP0 set, then the current kernel might
> eventually try to run userspace with the old kernel's RSP0 value.

That's why the init code needs to write every control MSR, even if it's
filling in 0 because it doesn't want to use the feature.

The job of the init code is to go from an unknown state into a good
state, not from the INIT/#RESET state.

Or someone could finally write down an ABI for Kexec which details who's
responsibility it is :)

~Andrew