vm-virtio: Detect if a tap interface supports multiqueue

By detecting if an existing tap interface supports multiqueue, we now
have the information to determine if the command line parameters
regarding the number of queues is correct.

Fixes #738

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vm-virtio/src/net_util.rs

@@ -7,6 +7,7 @@ use super::{DescriptorChain, DeviceEventT, Queue};
 use arc_swap::ArcSwap;
 use net_util::{MacAddr, Tap, TapError};
 use std::cmp;
+use std::fs;
 use std::io::{self, Write};
 use std::mem;
 use std::net::Ipv4Addr;
@@ -58,6 +59,8 @@ unsafe impl ByteValued for VirtioNetConfig {}
 
 #[derive(Debug)]
 pub enum Error {
+    /// Failed to convert an hexadecimal string into an integer.
+    ConvertHexStringToInt(std::num::ParseIntError),
     /// Read process MQ.
     FailedProcessMQ,
     /// Read queue failed.
@@ -70,10 +73,14 @@ pub enum Error {
     InvalidDesc,
     /// Invalid queue pairs number
     InvalidQueuePairsNum,
+    /// Error related to the multiqueue support.
+    MultiQueueSupport(String),
     /// No memory passed in.
     NoMemory,
     /// No ueue pairs nummber.
     NoQueuePairsNum,
+    /// Failed to read the TAP flags from sysfs.
+    ReadSysfsTunFlags(io::Error),
     /// Open tap device failed.
     TapOpen(TapError),
     /// Setting tap IP failed.
@@ -452,6 +459,26 @@ fn vnet_hdr_len() -> usize {
     mem::size_of::<virtio_net_hdr_v1>()
 }
 
+fn check_mq_support(if_name: &Option<&str>, queue_pairs: usize) -> Result<()> {
+    if let Some(tap_name) = if_name {
+        let mq = queue_pairs > 1;
+        let path = format!("/sys/class/net/{}/tun_flags", tap_name);
+        let tun_flags_str = fs::read_to_string(path).map_err(Error::ReadSysfsTunFlags)?;
+        let tun_flags = u32::from_str_radix(tun_flags_str.trim().trim_start_matches("0x"), 16)
+            .map_err(Error::ConvertHexStringToInt)?;
+        if (tun_flags & net_gen::IFF_MULTI_QUEUE != 0) && !mq {
+            return Err(Error::MultiQueueSupport(String::from(
+                "TAP interface supports MQ while device does not",
+            )));
+        } else if (tun_flags & net_gen::IFF_MULTI_QUEUE == 0) && mq {
+            return Err(Error::MultiQueueSupport(String::from(
+                "Device supports MQ while TAP interface does not",
+            )));
+        }
+    }
+    Ok(())
+}
+
 /// Create a new virtio network device with the given IP address and
 /// netmask.
 pub fn open_tap(
@@ -465,6 +492,13 @@ pub fn open_tap(
     let vnet_hdr_size = vnet_hdr_len() as i32;
     let flag = net_gen::TUN_F_CSUM | net_gen::TUN_F_UFO | net_gen::TUN_F_TSO4 | net_gen::TUN_F_TSO6;
 
+    // In case the tap interface already exists, check if the number of
+    // queues is appropriate. The tap might not support multiqueue while
+    // the number of queues indicates the user expects multiple queues, or
+    // on the contrary, the tap might support multiqueue while the number
+    // of queues indicates the user doesn't expect multiple queues.
+    check_mq_support(&if_name, num_rx_q)?;
+
     for i in 0..num_rx_q {
         let tap: Tap;
         if i == 0 {