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: <45d4edfd-d3d9-4115-9ec3-947630b06ce0@intel.com>
Date: Wed, 8 Jan 2025 14:31:19 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Andrew Cooper <andrew.cooper3@...rix.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 1/8/25 13:10, Andrew Cooper wrote:
> 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.

Right, so I think the quoted "Another option..." comment needs to get
zapped. Because it's not really another option. Discussing alternatives
is better left for changelogs anyway, not code comments.