Re: [PATCH v2 7/8] vsock/virtio: increase RX buffer size to 64 KiB

From: Jason Wang
Date: Mon May 13 2019 - 23:39:52 EST

On 2019/5/14 äå1:51, Stefano Garzarella wrote:
On Mon, May 13, 2019 at 06:01:52PM +0800, Jason Wang wrote:
On 2019/5/10 äå8:58, Stefano Garzarella wrote:
In order to increase host -> guest throughput with large packets,
we can use 64 KiB RX buffers.

Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
include/linux/virtio_vsock.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
index 84b72026d327..5a9d25be72df 100644
--- a/include/linux/virtio_vsock.h
+++ b/include/linux/virtio_vsock.h
@@ -10,7 +10,7 @@
#define VIRTIO_VSOCK_DEFAULT_BUF_SIZE (1024 * 256)
#define VIRTIO_VSOCK_MAX_PKT_BUF_SIZE (1024 * 64)

We probably don't want such high order allocation. It's better to switch to
use order 0 pages in this case. See add_recvbuf_big() for virtio-net. If we
get datapath unified, we will get more stuffs set.
IIUC, you are suggesting to allocate only pages and put them in a
scatterlist, then add them to the virtqueue.

Is it correct?

Yes since you are using:

ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ pkt->buf = kmalloc(buf_len, GFP_KERNEL);
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (!pkt->buf) {
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ virtio_transport_free_pkt(pkt);

This is likely to fail when the memory is fragmented which is kind of fragile.

The issue that I have here, is that the virtio-vsock guest driver, see
virtio_vsock_rx_fill(), allocates a struct virtio_vsock_pkt that
contains the room for the header, then allocates the buffer for the payload.
At this point it fills the scatterlist with the &virtio_vsock_pkt.hdr and the
buffer for the payload.

This part should be fine since what is needed is just adding more pages to sg[] and call virtuqeueu_add_sg().

Changing this will require several modifications, and if we get datapath
unified, I'm not sure it's worth it.
Of course, if we leave the datapaths separated, I'd like to do that later.

What do you think?

For the driver it self, it should not be hard. But I think you mean the issue of e.g virtio_vsock_pkt itself which doesn't support sg. For short time, maybe we can use kvec instead.