Peter Zijlstra [off-list ref] writes:

On Thu, Jul 23, 2020 at 03:17:04PM +0200, Thomas Gleixner wrote:

quoted

  2) Instruction synchronization

     Trying to do instruction synchronization delayed is a clear recipe
     for hard to diagnose failures. Just because it blew not up in your
     face does not make it correct in any way. It's broken by design and
     violates _all_ rules of safe instruction patching and introduces a
     complete trainwreck in x86 NMI processing.

     If you really think that this is correct, then please have at least
     the courtesy to come up with a detailed and precise argumentation
     why this is a valid approach.

     While writing that up you surely will find out why it is not.

So delaying the sync_core() IPIs for kernel text patching _might_ be
possible, but it very much wants to be a separate patchset and not
something hidden inside a 'gem' like this.

I'm not saying it's impossible, but the proposed hack is definitely
beyond broken and you really don't want to be the one who has to mop up
the pieces later.

Thanks,

        tglx