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[intel] Push new RX descriptors in batches 

Inside a virtual machine, writing the RX ring tail pointer may incur a
substantial overhead of processing inside the hypervisor.  Minimise
this overhead by writing the tail pointer once per batch of
descriptors, rather than once per descriptor.

Profiling under qemu-kvm (version 1.6.2) shows that this reduces the
amount of time taken to refill the RX descriptor ring by around 90%.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
pull/17/head
Michael Brown 2014-04-27 22:35:48 +01:00
parent 8a3dcefc0c
commit b2c7b6a85e
1 changed files with 13 additions and 8 deletions
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21
src/drivers/net/intel.c
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@ -456,19 +456,20 @@ void intel_refill_rx ( struct intel_nic *intel ) {
unsigned int rx_idx;
unsigned int rx_tail;
physaddr_t address;
unsigned int refilled = 0;
/* Refill ring */
while ( ( intel->rx.prod - intel->rx.cons ) < INTEL_RX_FILL ) {
/* Allocate I/O buffer */
iobuf = alloc_iob ( INTEL_RX_MAX_LEN );
if ( ! iobuf ) {
/* Wait for next refill */
return;
break;
}
/* Get next receive descriptor */
rx_idx = ( intel->rx.prod++ % INTEL_NUM_RX_DESC );
rx_tail = ( intel->rx.prod % INTEL_NUM_RX_DESC );
rx = &intel->rx.desc[rx_idx];
/* Populate receive descriptor */
@ -477,20 +478,24 @@ void intel_refill_rx ( struct intel_nic *intel ) {
rx->length = 0;
rx->status = 0;
rx->errors = 0;
wmb();
/* Record I/O buffer */
assert ( intel->rx_iobuf[rx_idx] == NULL );
intel->rx_iobuf[rx_idx] = iobuf;
/* Push descriptor to card */
profile_start ( &intel_vm_refill_profiler );
writel ( rx_tail, intel->regs + intel->rx.reg + INTEL_xDT );
profile_stop ( &intel_vm_refill_profiler );
DBGC2 ( intel, "INTEL %p RX %d is [%llx,%llx)\n", intel, rx_idx,
( ( unsigned long long ) address ),
( ( unsigned long long ) address + INTEL_RX_MAX_LEN ) );
refilled++;
}
/* Push descriptors to card, if applicable */
if ( refilled ) {
wmb();
rx_tail = ( intel->rx.prod % INTEL_NUM_RX_DESC );
profile_start ( &intel_vm_refill_profiler );
writel ( rx_tail, intel->regs + intel->rx.reg + INTEL_xDT );
profile_stop ( &intel_vm_refill_profiler );
}
}