From f42a0fd13bd281811e7457b28d939c8e8b808868 Mon Sep 17 00:00:00 2001 From: Jorgen Hansen Date: Thu, 12 Nov 2015 01:29:32 -0800 Subject: [PATCH] VMCI: Use 32bit atomics for queue headers on X86_32 This change restricts the reading and setting of the head and tail pointers on 32bit X86 to 32bit for both correctness and performance reasons. On uniprocessor X86_32, the atomic64_read may be implemented as a non-locked cmpxchg8b. This may result in updates to the pointers done by the VMCI device being overwritten. On MP systems, there is no such correctness issue, but using 32bit atomics avoids the overhead of the locked 64bit operation. All this is safe because the queue size on 32bit systems will never exceed a 32bit value. Reviewed-by: Thomas Hellstrom Signed-off-by: Jorgen Hansen Signed-off-by: Greg Kroah-Hartman --- drivers/misc/vmw_vmci/vmci_driver.c | 2 +- include/linux/vmw_vmci_defs.h | 43 ++++++++++++++++++++++++++--- 2 files changed, 40 insertions(+), 5 deletions(-) diff --git a/drivers/misc/vmw_vmci/vmci_driver.c b/drivers/misc/vmw_vmci/vmci_driver.c index b823f9a6e464..896be150e28f 100644 --- a/drivers/misc/vmw_vmci/vmci_driver.c +++ b/drivers/misc/vmw_vmci/vmci_driver.c @@ -113,5 +113,5 @@ module_exit(vmci_drv_exit); MODULE_AUTHOR("VMware, Inc."); MODULE_DESCRIPTION("VMware Virtual Machine Communication Interface."); -MODULE_VERSION("1.1.3.0-k"); +MODULE_VERSION("1.1.4.0-k"); MODULE_LICENSE("GPL v2"); diff --git a/include/linux/vmw_vmci_defs.h b/include/linux/vmw_vmci_defs.h index 65ac54c61c18..1bd31a38c51e 100644 --- a/include/linux/vmw_vmci_defs.h +++ b/include/linux/vmw_vmci_defs.h @@ -733,6 +733,41 @@ static inline void *vmci_event_data_payload(struct vmci_event_data *ev_data) return (void *)vmci_event_data_const_payload(ev_data); } +/* + * Helper to read a value from a head or tail pointer. For X86_32, the + * pointer is treated as a 32bit value, since the pointer value + * never exceeds a 32bit value in this case. Also, doing an + * atomic64_read on X86_32 uniprocessor systems may be implemented + * as a non locked cmpxchg8b, that may end up overwriting updates done + * by the VMCI device to the memory location. On 32bit SMP, the lock + * prefix will be used, so correctness isn't an issue, but using a + * 64bit operation still adds unnecessary overhead. + */ +static inline u64 vmci_q_read_pointer(atomic64_t *var) +{ +#if defined(CONFIG_X86_32) + return atomic_read((atomic_t *)var); +#else + return atomic64_read(var); +#endif +} + +/* + * Helper to set the value of a head or tail pointer. For X86_32, the + * pointer is treated as a 32bit value, since the pointer value + * never exceeds a 32bit value in this case. On 32bit SMP, using a + * locked cmpxchg8b adds unnecessary overhead. + */ +static inline void vmci_q_set_pointer(atomic64_t *var, + u64 new_val) +{ +#if defined(CONFIG_X86_32) + return atomic_set((atomic_t *)var, (u32)new_val); +#else + return atomic64_set(var, new_val); +#endif +} + /* * Helper to add a given offset to a head or tail pointer. Wraps the * value of the pointer around the max size of the queue. @@ -741,14 +776,14 @@ static inline void vmci_qp_add_pointer(atomic64_t *var, size_t add, u64 size) { - u64 new_val = atomic64_read(var); + u64 new_val = vmci_q_read_pointer(var); if (new_val >= size - add) new_val -= size; new_val += add; - atomic64_set(var, new_val); + vmci_q_set_pointer(var, new_val); } /* @@ -758,7 +793,7 @@ static inline u64 vmci_q_header_producer_tail(const struct vmci_queue_header *q_header) { struct vmci_queue_header *qh = (struct vmci_queue_header *)q_header; - return atomic64_read(&qh->producer_tail); + return vmci_q_read_pointer(&qh->producer_tail); } /* @@ -768,7 +803,7 @@ static inline u64 vmci_q_header_consumer_head(const struct vmci_queue_header *q_header) { struct vmci_queue_header *qh = (struct vmci_queue_header *)q_header; - return atomic64_read(&qh->consumer_head); + return vmci_q_read_pointer(&qh->consumer_head); } /* -- 2.30.2