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

From: "Paul E. McKenney" <[email-masked]>
Date: Thu, 20 Apr 2017 17:38:34 -0700

On Thu, Apr 20, 2017 at 05:28:32PM +0200, Paolo Bonzini wrote:
> 
> 
> On 20/04/2017 05:40, Michael Ellerman wrote:
> > "Paul E. McKenney" <[email-masked]> 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.
> 
> Michael, if you want to move the two srcu structs at the end of struct
> kvm, that would be fine by me.  Please send a patch yourself so you can
> test it on PPC.  Thanks,

On the off-chance that it is at all helpful, I have added the commit
shown below to -rcu.  If it helps, I am happy to push this, but am
just as happy to drop it in favor of some other fix.  If at all possible,
I would like to get this into the upcoming merge window.

							Thanx, Paul