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

[Xin Li]

On 1/9/2025 1:46 AM, H. Peter Anvin wrote:
> On January 8, 2025 12:14:18 PM PST, Dave Hansen <dave.hansen@xxxxxxxxx> 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.
>
> Yeah, it is a nonstarter. I believe Xin was trying to document the problems with *not* doing this, so someone doesn't have the bright idea to "optimize" this later.

exactly ;)