pxeboot: improve and simplify rx handling

This pushes the bulk of the rx servicing into a single loop that's only
slightly convoluted, and it addresses a problem with rx handling in the
process.  If we hit a tx interrupt while we're processing, we'd
previously drop the frame on the floor completely and ultimately
timeout, increasing boot time on particularly busy hosts as we keep
having to backoff and resend.

After this patch, we don't seem to hit timeouts at all on zoo anymore
though loading a 27M kernel is still relatively slow (~1m20s).

Sponsored By:	National Bureau of Economic Research
Sponsored by:	Klara, Inc.

(cherry picked from commit 3daa8e1)

stand/i386/libi386/pxe.c

@@ -30,6 +30,8 @@
 __FBSDID("$FreeBSD$");
 
 #include <stand.h>
+#include <errno.h>
+#include <stdbool.h>
 #include <stddef.h>
 #include <string.h>
 #include <stdarg.h>
@@ -432,91 +434,144 @@ pxe_netif_init(struct iodesc *desc, void *machdep_hint)
 }
 
 static int
-pxe_netif_receive(void **pkt)
+pxe_netif_receive_isr(t_PXENV_UNDI_ISR *isr, void **pkt, ssize_t *retsize)
 {
-	t_PXENV_UNDI_ISR *isr;
+	static bool data_pending;
 	char *buf, *ptr, *frame;
 	size_t size, rsize;
 
-	isr = bio_alloc(sizeof(*isr));
-	if (isr == NULL)
-		return (-1);
+	buf = NULL;
+	size = rsize = 0;
+
+	/*
+	 * We can save ourselves the next two pxe calls because we already know
+	 * we weren't done grabbing everything.
+	 */
+	if (data_pending) {
+		data_pending = false;
+		goto nextbuf;
+	}
 
+	/*
+	 * We explicitly don't check for OURS/NOT_OURS as a result of START;
+	 * it's been reported that some cards are known to mishandle these.
+	 */
 	bzero(isr, sizeof(*isr));
 	isr->FuncFlag = PXENV_UNDI_ISR_IN_START;
 	pxe_call(PXENV_UNDI_ISR, isr);
+	/* We could translate Status... */
 	if (isr->Status != 0) {
-		bio_free(isr, sizeof(*isr));
-		return (-1);
+		return (ENXIO);
 	}
 
 	bzero(isr, sizeof(*isr));
 	isr->FuncFlag = PXENV_UNDI_ISR_IN_PROCESS;
 	pxe_call(PXENV_UNDI_ISR, isr);
 	if (isr->Status != 0) {
-		bio_free(isr, sizeof(*isr));
-		return (-1);
+		return (ENXIO);
 	}
-
-	while (isr->FuncFlag == PXENV_UNDI_ISR_OUT_TRANSMIT) {
+	if (isr->FuncFlag == PXENV_UNDI_ISR_OUT_BUSY) {
 		/*
-		 * Wait till transmit is done.
+		 * Let the caller decide if we need to be restarted.  It will
+		 * currently blindly restart us, but it could check timeout in
+		 * the future.
 		 */
-		bzero(isr, sizeof(*isr));
-		isr->FuncFlag = PXENV_UNDI_ISR_IN_GET_NEXT;
-		pxe_call(PXENV_UNDI_ISR, isr);
-		if (isr->Status != 0 ||
-		    isr->FuncFlag == PXENV_UNDI_ISR_OUT_DONE) {
-			bio_free(isr, sizeof(*isr));
-			return (-1);
-		}
+		return (ERESTART);
 	}
 
-	while (isr->FuncFlag != PXENV_UNDI_ISR_OUT_RECEIVE) {
-		if (isr->Status != 0 ||
-		    isr->FuncFlag == PXENV_UNDI_ISR_OUT_DONE) {
-			bio_free(isr, sizeof(*isr));
-			return (-1);
+	/*
+	 * By design, we'll hardly ever hit this terminal condition unless we
+	 * pick up nothing but tx interrupts here.  More frequently, we will
+	 * process rx buffers until we hit the terminal condition in the middle.
+	 */
+	while (isr->FuncFlag != PXENV_UNDI_ISR_OUT_DONE) {
+		/*
+		 * This might have given us PXENV_UNDI_ISR_OUT_TRANSMIT, in
+		 * which case we can just disregard and move on to the next
+		 * buffer/frame.
+		 */
+		if (isr->FuncFlag != PXENV_UNDI_ISR_OUT_RECEIVE)
+			goto nextbuf;
+
+		if (buf == NULL) {
+			/*
+			 * Grab size from the first Frame that we picked up,
+			 * allocate an rx buf to hold.  Careful here, as we may
+			 * see a fragmented frame that's spread out across
+			 * multiple GET_NEXT calls.
+			 */
+			size = isr->FrameLength;
+			buf = malloc(size + ETHER_ALIGN);
+			if (buf == NULL)
+				return (ENOMEM);
+
+			ptr = buf + ETHER_ALIGN;
 		}
-		bzero(isr, sizeof(*isr));
-		isr->FuncFlag = PXENV_UNDI_ISR_IN_GET_NEXT;
-		pxe_call(PXENV_UNDI_ISR, isr);
-	}
-
-	size = isr->FrameLength;
-	buf = malloc(size + ETHER_ALIGN);
-	if (buf == NULL) {
-		bio_free(isr, sizeof(*isr));
-		return (-1);
-	}
-	ptr = buf + ETHER_ALIGN;
-	rsize = 0;
 
-	while (rsize < size) {
 		frame = (char *)((uintptr_t)isr->Frame.segment << 4);
 		frame += isr->Frame.offset;
 		bcopy(PTOV(frame), ptr, isr->BufferLength);
 		ptr += isr->BufferLength;
 		rsize += isr->BufferLength;
 
+		/*
+		 * Stop here before we risk catching the start of another frame.
+		 * It would be nice to continue reading until we actually get a
+		 * PXENV_UNDI_ISR_OUT_DONE, but our network stack in libsa isn't
+		 * suitable for reading more than one packet at a time.
+		 */
+		if (rsize >= size) {
+			data_pending = true;
+			break;
+		}
+
+nextbuf:
 		bzero(isr, sizeof(*isr));
 		isr->FuncFlag = PXENV_UNDI_ISR_IN_GET_NEXT;
 		pxe_call(PXENV_UNDI_ISR, isr);
 		if (isr->Status != 0) {
-			bio_free(isr, sizeof(*isr));
 			free(buf);
-			return (-1);
+			return (ENXIO);
 		}
+	}
 
-		/* Did we got another update? */
-		if (isr->FuncFlag == PXENV_UNDI_ISR_OUT_RECEIVE)
-			continue;
-		break;
+	/*
+	 * We may have never picked up a frame at all (all tx), in which case
+	 * the caller should restart us.
+	 */
+	if (rsize == 0) {
+		return (ERESTART);
 	}
 
 	*pkt = buf;
+	*retsize = rsize;
+	return (0);
+}
+
+static int
+pxe_netif_receive(void **pkt, ssize_t *size)
+{
+	t_PXENV_UNDI_ISR *isr;
+	int ret;
+
+	isr = bio_alloc(sizeof(*isr));
+	if (isr == NULL)
+		return (ENOMEM);
+
+	/*
+	 * This completely ignores the timeout specified in pxe_netif_get(), but
+	 * we shouldn't be running long enough here for that to make a
+	 * difference.
+	 */
+	for (;;) {
+		/* We'll only really re-enter for PXENV_UNDI_ISR_OUT_BUSY. */
+		ret = pxe_netif_receive_isr(isr, pkt, size);
+		if (ret != ERESTART)
+			break;
+	}
+
 	bio_free(isr, sizeof(*isr));
-	return (rsize);
+	return (ret);
 }
 
 static ssize_t
@@ -525,16 +580,19 @@ pxe_netif_get(struct iodesc *desc, void **pkt, time_t timeout)
 	time_t t;
 	void *ptr;
 	int ret = -1;
+	ssize_t size;
 
 	t = getsecs();
+	size = 0;
 	while ((getsecs() - t) < timeout) {
-		ret = pxe_netif_receive(&ptr);
+		ret = pxe_netif_receive(&ptr, &size);
 		if (ret != -1) {
 			*pkt = ptr;
 			break;
 		}
 	}
-	return (ret);
+
+	return (ret == 0 ? size : -1);
 }
 
 static ssize_t