Re: [PATCH RFC V3 6/9] x86/entry: Pass irqentry_state_t by reference

[Ira Weiny]

On Mon, Oct 19, 2020 at 11:32:50AM +0200, Thomas Gleixner wrote:
> On Sun, Oct 18 2020 at 22:37, Ira Weiny wrote:
> > On Fri, Oct 16, 2020 at 02:55:21PM +0200, Thomas Gleixner wrote:
> >> Subject: x86/entry: Move nmi entry/exit into common code
> >> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >> Date: Fri, 11 Sep 2020 10:09:56 +0200
> >> 
> >> Add blurb here.
> >
> > How about:
> >
> > To prepare for saving PKRS values across NMI's we lift the
> > idtentry_[enter|exit]_nmi() to the common code.  Rename them to
> > irqentry_nmi_[enter|exit]() to reflect the new generic nature and store the
> > state in the same irqentry_state_t structure as the other irqentry_*()
> > functions.  Finally, differentiate the state being stored between the NMI and
> > IRQ path by adding 'lockdep' to irqentry_state_t.
> 
> No. This has absolutely nothing to do with PKRS. It's a cleanup valuable
> by itself and that's how it should have been done right away.
> 
> So the proper changelog is:
> 
>   Lockdep state handling on NMI enter and exit is nothing specific to
>   X86. It's not any different on other architectures. Also the extra
>   state type is not necessary, irqentry_state_t can carry the necessary
>   information as well.
> 
>   Move it to common code and extend irqentry_state_t to carry lockdep
>   state.

Ok sounds good, thanks.

> 
> >> --- a/include/linux/entry-common.h
> >> +++ b/include/linux/entry-common.h
> >> @@ -343,6 +343,7 @@ void irqentry_exit_to_user_mode(struct p
> >>  #ifndef irqentry_state
> >>  typedef struct irqentry_state {
> >>  	bool	exit_rcu;
> >> +	bool	lockdep;
> >>  } irqentry_state_t;
> >
> > Building on what Peter said do you agree this should be made into a union?
> >
> > It may not be strictly necessary in this patch but I think it would reflect the
> > mutual exclusivity better and could be changed easy enough in the follow on
> > patch which adds the pkrs state.
> 
> Why the heck should it be changed in a patch which adds something
> completely different?

Because the PKRS stuff is used in both NMI and IRQ state.

> 
> Either it's mutually exclusive or not and if so it want's to be done in
> this patch and not in a change which extends the struct for other
> reasons.

Sorry, let me clarify.  After this patch we have.

typedef union irqentry_state {
	bool	exit_rcu;
	bool	lockdep;
} irqentry_state_t;

Which reflects the mutual exclusion of the 2 variables.

But then when the pkrs stuff is added the union changes back to a structure and
looks like this.

typedef struct irqentry_state {
#ifdef CONFIG_ARCH_HAS_SUPERVISOR_PKEYS
        u32 pkrs;
        u32 thread_pkrs;
#endif  
	union {
		bool    exit_rcu;
		bool	lockdep;
	};
} irqentry_state_t;

Because the pkrs information is in addition to exit_rcu OR lockdep.

So this is what I meant by 'could be changed easy enough in the follow on
patch'.

Is that clear?

Ira