Re: [PATCH] kho: add support for deferred struct page init

[Mike Rapoport <rppt@kernel.org>]

On Tue, Dec 30, 2025 at 11:18:12AM -0500, Pasha Tatashin wrote:
> On Tue, Dec 30, 2025 at 11:16 AM Mike Rapoport <rppt@kernel.org> wrote:
> >
> > On Tue, Dec 30, 2025 at 11:05:05AM -0500, Pasha Tatashin wrote:
> > > On Mon, Dec 29, 2025 at 4:03 PM Pratyush Yadav <pratyush@kernel.org> wrote:
> > > >
> > > > The magic is purely sanity checking. It is not used to decide anything
> > > > other than to make sure this is actually a KHO page. I don't intend to
> > > > change that. My point is, if we make sure the KHO pages are properly
> > > > initialized during MM init, then restoring can actually be a very cheap
> > > > operation, where you only do the sanity checking. You can even put the
> > > > magic check behind CONFIG_KEXEC_HANDOVER_DEBUG if you want, but I think
> > > > it is useful enough to keep in production systems too.
> > >
> > > It is part of a critical hotpath during blackout, should really be
> > > behind CONFIG_KEXEC_HANDOVER_DEBUG
> >
> > Do you have the numbers? ;-)
> 
> The fastest reboot we can achieve is ~0.4s on ARM

I meant the difference between assigning info.magic and skipping it.

> (shutdown+purgatory+boot), let's not add anything to regress, as every
> microsecond counts during blackout.

Any added functionality adds cycles, this is inevitable. And neither KHO
nor LUO are near the completion, so we'll have to add functionality to both
of them. And the added functionality should be correct first and foremost.
And magic sanity check seems pretty useful and presumably cheap enough to
always keep it unless you see a real slowdown because of it.
 
> Pasha

-- 
Sincerely yours,
Mike.