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

[Michael Ellerman]

"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.

cheers