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SubjectRe: powerpc KVM build break in linux-next (was Re: [PATCH tip/core/rcu 40/40] srcu: Parallelize callback handling)
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On 20/04/2017 05:40, Michael Ellerman wrote:
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> 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,

Paolo

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