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

[Dave Hansen]

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.