From: Michael Brown <mcb30@ipxe.org>
Date: Sun, 27 Apr 2014 21:35:48 +0000 (+0100)
Subject: [intel] Push new RX descriptors in batches
X-Git-Tag: v1.20.1~1215
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=b2c7b6a85e8078d5b6692156cb9850590a8ff6aa;p=thirdparty%2Fipxe.git

[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>
---

diff --git a/src/drivers/net/intel.c b/src/drivers/net/intel.c
index 5abcdd7f8..6684bdbd9 100644
--- a/src/drivers/net/intel.c
+++ b/src/drivers/net/intel.c
@@ -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 );
 	}
 }