Re: [PATCH v4 29/39] unwind_user/deferred: Add unwind cache

[Peter Zijlstra]

On Wed, Jan 22, 2025 at 02:36:25PM -0800, Josh Poimboeuf wrote:
> On Wed, Jan 22, 2025 at 02:57:00PM +0100, Peter Zijlstra wrote:
> > On Tue, Jan 21, 2025 at 06:31:21PM -0800, Josh Poimboeuf wrote:
> > > Cache the results of the unwind to ensure the unwind is only performed
> > > once, even when called by multiple tracers.
> > > 
> > > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > > ---
> > >  include/linux/unwind_deferred_types.h |  8 +++++++-
> > >  kernel/unwind/deferred.c              | 26 ++++++++++++++++++++------
> > >  2 files changed, 27 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h
> > > index 9749824aea09..6f71a06329fb 100644
> > > --- a/include/linux/unwind_deferred_types.h
> > > +++ b/include/linux/unwind_deferred_types.h
> > > @@ -2,8 +2,14 @@
> > >  #ifndef _LINUX_UNWIND_USER_DEFERRED_TYPES_H
> > >  #define _LINUX_UNWIND_USER_DEFERRED_TYPES_H
> > >  
> > > -struct unwind_task_info {
> > > +struct unwind_cache {
> > >  	unsigned long		*entries;
> > > +	unsigned int		nr_entries;
> > > +	u64			cookie;
> > > +};
> > 
> > If you make the return to user path clear nr_entries you don't need a
> > second cookie field I think.
> 
> But if the NMI happens late in the exit-to-user path, with IRQs
> disabled, right before nr_entries gets cleared, the cache won't get
> used in the task work.
> 
> However I think we can clear it on entry-from-user.

Return to user runs with interrupts disabled, if an NMI hits that, it
will have to set TIF_NOTIFY_RESUME again and queue the IRQ work thing.
That self-IPI will hit the moment we do IRET (which is what re-enables
interrupts) and we're going back into the kernel.

Anyway, I suppose that is a long way of saying that you should be able
to do this on return to user.

But yes, enter-from-user should work too.