Re: powerpc KVM build break in linux-next (was Re: [PATCH tip/core/rcu 40/40] srcu: Parallelize callback handling)

[Paul E. McKenney]

On Thu, Apr 20, 2017 at 01:40:13PM +1000, Michael Ellerman wrote:
> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> writes:
> 
> > diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> > index f2b3bd6c6bc2..0400e211aa44 100644
> > --- a/include/linux/srcutree.h
> > +++ b/include/linux/srcutree.h
> > @@ -24,25 +24,75 @@
> ...
> > +/*
> > + * Per-SRCU-domain structure, similar in function to rcu_state.
> > + */
> >  struct srcu_struct {
> > -	unsigned long completed;
> > -	unsigned long srcu_gp_seq;
> > -	atomic_t srcu_exp_cnt;
> > -	struct srcu_array __percpu *per_cpu_ref;
> > -	spinlock_t queue_lock; /* protect ->srcu_cblist */
> > -	struct rcu_segcblist srcu_cblist;
> > +	struct srcu_node node[NUM_RCU_NODES];	/* Combining tree. */
> > +	struct srcu_node *level[RCU_NUM_LVLS + 1];
> > +						/* First node at each level. */
> > +	struct mutex srcu_cb_mutex;		/* Serialize CB preparation. */
> > +	spinlock_t gp_lock;			/* protect ->srcu_cblist */
> > +	struct mutex srcu_gp_mutex;		/* Serialize GP work. */
> > +	unsigned int srcu_idx;			/* Current rdr array element. */
> > +	unsigned long srcu_gp_seq;		/* Grace-period seq #. */
> > +	unsigned long srcu_gp_seq_needed;	/* Latest gp_seq needed. */
> > +	atomic_t srcu_exp_cnt;			/* # ongoing expedited GPs. */
> > +	struct srcu_data __percpu *sda;		/* Per-CPU srcu_data array. */
> > +	unsigned long srcu_barrier_seq;		/* srcu_barrier seq #. */
> > +	struct mutex srcu_barrier_mutex;	/* Serialize barrier ops. */
> > +	struct completion srcu_barrier_completion;
> > +						/* Awaken barrier rq at end. */
> > +	atomic_t srcu_barrier_cpu_cnt;		/* # CPUs not yet posting a */
> > +						/*  callback for the barrier */
> > +						/*  operation. */
> 
> This change seems to have had the non-obvious effect of breaking the
> powerpc KVM build.
> 
> Because struct kvm contains two srcu_structs which are located
> before the kvm_arch struct, the increase in size of srcu_struct has
> caused the offset from the start of struct kvm to kvm_arch to be too big
> for some of our asm.
> 
> struct kvm {
> 	spinlock_t mmu_lock;
> 	struct mutex slots_lock;
> 	struct mm_struct *mm; /* userspace tied to this vm */
> 	struct kvm_memslots *memslots[KVM_ADDRESS_SPACE_NUM];
> 	struct srcu_struct srcu;
> 	struct srcu_struct irq_srcu;
>         ...
> 	struct kvm_arch arch;
> 
> 
> Example error:
>   arch/powerpc/kvm/book3s_hv_rmhandlers.S:617: Error: operand out of range (0x000000000000b328 is not between 0xffffffffffff8000 and 0x0000000000007fff)
> 
> Where line 617 is:
>   lwz	r7,KVM_LPID(r9)
> 
> And the KVM_LPID constant comes from asm-offsets.s. The diff of old vs
> new is:
> 
>   -->KVM_LPID 17752 offsetof(struct kvm, arch.lpid)	 #
>   +->KVM_LPID 45864 offsetof(struct kvm, arch.lpid)	 #
> 
> 
> We can probably just fix it by changing the asm to keep the address of
> kvm_arch in a register, and then offset from that.

Ouch!

What do you need from me?

							Thanx, Paul