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vmx: reset virtio queue state on device reset

when a virtio device gets reset, we have to also reset the device
shadow indices: availableidx and usedidx. for extra safetly,
we also reset the buffer descriptor table addresses.

this is accomplished by adding a vioqreset(VIOQueue*) function
that brings the queue to its initial reset state.

this fixes non functional ethernet after reboot(8).

--- a/sys/src/cmd/vmx/virtio.c

+++ b/sys/src/cmd/vmx/virtio.c

@@ -311,6 +311,17 @@

 }

 static void

+vioqreset(VIOQueue *q)

+{

+	q->desc = nil;

+	q->avail = nil;

+	q->used = nil;

+	q->addr = 0;

+	q->availidx = 0;

+	q->usedidx = 0;

+}

+

+static void

 viodevstatset(VIODev *v, u32int val)

 {

 	int i;

@@ -325,6 +336,7 @@

 			qlock(&v->qu[i]);

 			while(v->qu[i].livebuf > 0)

 				rsleep(&v->qu[i].livebufrend);

+			vioqreset(&v->qu[i]);

 			qunlock(&v->qu[i]);

 		}

 	}else{

@@ -396,6 +408,7 @@

 	q->size = sz;

 	q->d = d;

 	q->notify = fn;

+	vioqreset(q);

 	return q;

 }