> Replace deprecated rte_atomic32 operations in the vmbus ring buffer > producer with stdatomic equivalents, and replace the smp_wmb + CAS-spin > publish with rte_wait_until_equal_32 + release-store. > > The two-cursor design is preserved: tbr->windex is the driver-private > reservation cursor that lets producers reserve slots concurrently without a > lock; vbr->windex is the host-visible commit cursor, updated in reservation > order so the host never observes windex pointing past unwritten data. This is > the lockless analogue of the spinlock-around- single-cursor pattern used by > the Linux (drivers/hv/ring_buffer.c > hv_ringbuffer_write) and FreeBSD (sys/dev/hyperv/vmbus/vmbus_br.c > vmbus_txbr_write) implementations of the same host contract. > > The memory ordering mirrors __rte_ring_headtail_move_head and > __rte_ring_update_tail in lib/ring/rte_ring_c11_pvt.h: relaxed wait for the > previous producer's commit, release-store to publish. The rte_smp_wmb > before the publish is folded into the release ordering on the store itself. > > The host-shared vbr->windex remains volatile uint32_t in the packed bufring > struct; the atomic qualifier is added via cast at the access site. The > (uintptr_t) > launder on the store-side cast suppresses a spurious misaligned-atomic > warning from the packed-struct attribute (windex is 4-byte aligned in > practice, > at offset 0 of a page-aligned struct). > > Signed-off-by: Stephen Hemminger <[email protected]>
Reviewed-by: Long Li <[email protected]> > --- > drivers/bus/vmbus/private.h | 2 +- > drivers/bus/vmbus/vmbus_bufring.c | 39 +++++++++++++++++-------------- > 2 files changed, 23 insertions(+), 18 deletions(-) > > diff --git a/drivers/bus/vmbus/private.h b/drivers/bus/vmbus/private.h index > 6efac86b77..6b7782724f 100644 > --- a/drivers/bus/vmbus/private.h > +++ b/drivers/bus/vmbus/private.h > @@ -25,7 +25,7 @@ extern int vmbus_logtype_bus; struct vmbus_br { > struct vmbus_bufring *vbr; > uint32_t dsize; > - uint32_t windex; /* next available location */ > + RTE_ATOMIC(uint32_t) windex; /* next available location */ > }; > > #define UIO_NAME_MAX 64 > diff --git a/drivers/bus/vmbus/vmbus_bufring.c > b/drivers/bus/vmbus/vmbus_bufring.c > index fcb97287dc..624fe8b6c5 100644 > --- a/drivers/bus/vmbus/vmbus_bufring.c > +++ b/drivers/bus/vmbus/vmbus_bufring.c > @@ -15,7 +15,7 @@ > #include <rte_tailq.h> > #include <rte_log.h> > #include <rte_malloc.h> > -#include <rte_atomic.h> > +#include <rte_stdatomic.h> > #include <rte_memory.h> > #include <rte_pause.h> > #include <rte_bus_vmbus.h> > @@ -114,6 +114,7 @@ vmbus_txbr_write(struct vmbus_br *tbr, const struct > iovec iov[], int iovlen, > uint32_t ring_size = tbr->dsize; > uint32_t old_windex, next_windex, windex, total; > uint64_t save_windex; > + bool success; > int i; > > total = 0; > @@ -121,17 +122,13 @@ vmbus_txbr_write(struct vmbus_br *tbr, const > struct iovec iov[], int iovlen, > total += iov[i].iov_len; > total += sizeof(save_windex); > > + /* Get current free location */ > + old_windex = rte_atomic_load_explicit(&tbr->windex, > + rte_memory_order_relaxed); > + > /* Reserve space in ring */ > do { > - uint32_t avail; > - > - /* Get current free location */ > - old_windex = tbr->windex; > - > - /* Prevent compiler reordering this with calculation */ > - rte_compiler_barrier(); > - > - avail = vmbus_br_availwrite(tbr, old_windex); > + uint32_t avail = vmbus_br_availwrite(tbr, old_windex); > > /* If not enough space in ring, then tell caller. */ > if (avail <= total) > @@ -139,8 +136,13 @@ vmbus_txbr_write(struct vmbus_br *tbr, const > struct iovec iov[], int iovlen, > > next_windex = vmbus_br_idxinc(old_windex, total, ring_size); > > - /* Atomic update of next write_index for other threads */ > - } while (!rte_atomic32_cmpset(&tbr->windex, old_windex, > next_windex)); > + /* Atomic update of next write_index for other threads > + * Can use weak since easy to recompute and retry. > + */ > + success = rte_atomic_compare_exchange_weak_explicit( > + &tbr->windex, &old_windex, next_windex, > + rte_memory_order_acquire, > rte_memory_order_relaxed); > + } while (unlikely(!success)); > > /* Space from old..new is now reserved */ > windex = old_windex; > @@ -157,12 +159,15 @@ vmbus_txbr_write(struct vmbus_br *tbr, const > struct iovec iov[], int iovlen, > /* The region reserved should match region used */ > RTE_ASSERT(windex == next_windex); > > - /* Ensure that data is available before updating host index */ > - rte_smp_wmb(); > + /* Wait for previous producer to publish their windex update */ > + rte_wait_until_equal_32(&vbr->windex, old_windex, > +rte_memory_order_relaxed); > > - /* Checkin for our reservation. wait for our turn to update host */ > - while (!rte_atomic32_cmpset(&vbr->windex, old_windex, > next_windex)) > - rte_pause(); > + /* Publish our windex update; prior data writes ordered via release. > + * windex is 4-byte aligned in practice (struct is page-aligned, windex > + * at offset 0); cast launders the packed-struct alignment-1 attribute. > + */ > + rte_atomic_store_explicit((volatile __rte_atomic uint32_t > *)(uintptr_t)&vbr->windex, > + next_windex, rte_memory_order_release); > > /* If host had read all data before this, then need to signal */ > *need_sig |= vmbus_txbr_need_signal(vbr, old_windex); > -- > 2.53.0
