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

[Xin Li]

On 1/8/2025 2:31 PM, Dave Hansen wrote:
> 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.

Will do!