From 8627656f55480669b125456d1e59f68966e64415 Mon Sep 17 00:00:00 2001
From: Sebastien Boeuf <sebastien.boeuf@intel.com>
Date: Wed, 12 Feb 2020 09:36:26 +0100
Subject: [PATCH] net_util: Provide more accurate error messages

In order to identify better the problem whenever an existing TAP is
provided, this patch enhances the error returned for an easier
diagnosis of the problem.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
---
 net_util/src/tap.rs | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/net_util/src/tap.rs b/net_util/src/tap.rs
index 310e879bb..fe0ada459 100644
--- a/net_util/src/tap.rs
+++ b/net_util/src/tap.rs
@@ -20,8 +20,12 @@ use vmm_sys_util::ioctl::{ioctl_with_mut_ref, ioctl_with_ref, ioctl_with_val};
 pub enum Error {
     /// Couldn't open /dev/net/tun.
     OpenTun(IoError),
-    /// Unable to create tap interface.
-    CreateTap(IoError),
+    /// Unable to configure tap interface.
+    ConfigureTap(IoError),
+    /// Unable to retrieve features.
+    GetFeatures(IoError),
+    /// Missing multiqueue support in the kernel.
+    MultiQueueKernelSupport,
     /// ioctl failed.
     IoctlError(IoError),
     /// Failed to create a socket.
@@ -96,6 +100,20 @@ impl Tap {
         // We just checked that the fd is valid.
         let tuntap = unsafe { File::from_raw_fd(fd) };
 
+        // Let's validate some features before going any further.
+        // ioctl is safe since we call it with a valid tap fd and check the return
+        // value.
+        let mut features = 0;
+        let ret = unsafe { ioctl_with_mut_ref(&tuntap, net_gen::TUNGETFEATURES(), &mut features) };
+        if ret < 0 {
+            return Err(Error::GetFeatures(IoError::last_os_error()));
+        }
+
+        // Check if the user parameters match the kernel support for MQ
+        if (features & net_gen::IFF_MULTI_QUEUE == 0) && num_queue_pairs > 1 {
+            return Err(Error::MultiQueueKernelSupport);
+        }
+
         // This is pretty messy because of the unions used by ifreq. Since we
         // don't call as_mut on the same union field more than once, this block
         // is safe.
@@ -115,9 +133,8 @@ impl Tap {
         // ioctl is safe since we call it with a valid tap fd and check the return
         // value.
         let ret = unsafe { ioctl_with_mut_ref(&tuntap, net_gen::TUNSETIFF(), &mut ifreq) };
-
         if ret < 0 {
-            return Err(Error::CreateTap(IoError::last_os_error()));
+            return Err(Error::ConfigureTap(IoError::last_os_error()));
         }
 
         let if_name_temp = unsafe { *ifreq.ifr_ifrn.ifrn_name.as_ref() };