[Qemu-devel] [PATCH 1/1] virtio-mmio: return the max queue num of virtio-mmio with initial value

[Wei Huang <wei@redhat.com>]

Recently we found that virtio-console devices consumes lots AArch64 guest
memory, roughly 1GB with 8 devices. After debugging, it turns out that lots
of factors contribute to this problem: i) guest PAGE_SIZE=64KB, ii)
virtio-mmio based devices, and iii) virtio-console device. Here is the
detailed analysis:

1. First, during initialization, virtio-mmio driver in guest pokes vq
   size by reading VIRTIO_MMIO_QUEUE_NUM_MAX (see virtio_mmio.c file).
2. QEMU returns VIRTQUEUE_MAX_SIZE (1024) to guest VM; And virtio-mmio uses
   it as the default vq size.
3. virtio-console driver allocates vring buffers based on this value (see
   add_inbuf() function of virtio_console.c file). Because PAGE_SIZE=64KB,
   ~64MB is allocated for each virtio-console vq.

This patch addresses the problem by returning the iniatlized vring size
when VM queries QEMU about VIRTIO_MMIO_QUEUE_NUM_MAX. This is similar to
virtio-pci's approach. By doing this, the vq memory consumption is reduced
substantially.

Signed-off-by: Wei Huang <wei@redhat.com>
---
 hw/virtio/virtio-mmio.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/hw/virtio/virtio-mmio.c b/hw/virtio/virtio-mmio.c
index 10123f3..27840fe 100644
--- a/hw/virtio/virtio-mmio.c
+++ b/hw/virtio/virtio-mmio.c
@@ -93,6 +93,7 @@ static uint64_t virtio_mmio_read(void *opaque, hwaddr offset, unsigned size)
 {
     VirtIOMMIOProxy *proxy = (VirtIOMMIOProxy *)opaque;
     VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
+    uint64_t queue_num;
 
     DPRINTF("virtio_mmio_read offset 0x%x\n", (int)offset);
 
@@ -149,10 +150,8 @@ static uint64_t virtio_mmio_read(void *opaque, hwaddr offset, unsigned size)
         }
         return proxy->host_features;
     case VIRTIO_MMIO_QUEUENUMMAX:
-        if (!virtio_queue_get_num(vdev, vdev->queue_sel)) {
-            return 0;
-        }
-        return VIRTQUEUE_MAX_SIZE;
+        queue_num = virtio_queue_get_num(vdev, vdev->queue_sel);
+        return queue_num;
     case VIRTIO_MMIO_QUEUEPFN:
         return virtio_queue_get_addr(vdev, vdev->queue_sel)
             >> proxy->guest_page_shift;
-- 
1.8.3.1