The VirtQueueElement is a very big structure (>48k!), since it will be
initialzed by virtqueue_pop, we can save the expensive zeroing here.
This saves a few microseconds per request in my test:
[fio-test] rw bs iodepth jobs bw iops latency
--------------------------------------------------------------------------------------------
Before read 4k 1 1 110 28269 34
After read 4k 1 1 131 33745 28
Whereas,
virtio-blk read 4k 1 1 217 55673 16
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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