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Re: [libvirt PATCH v2 1/2] util: replace macvtap name reservation bitmap with a simple counter

On 8/26/20 9:00 AM, Michal Privoznik wrote:
On 8/26/20 7:22 AM, Laine Stump wrote:
There have been some reports that, due to libvirt always trying to
assign the lowest numbered macvtap / tap device name possible, a new
guest would sometimes be started using the same tap device name as
previously used by another guest that is in the process of being
destroyed *as the new guest is starting.

In some cases this has led to, for example, the old guest's
qemuProcessStop() code deleting a port from an OVS switch that had
just been re-added by the new guest (because the port name is based on
only the device name using the port). Similar problems can happen (and
I believe have) with nwfilter rules and bandwidth rules (which are
both instantiated based on the name of the tap device).

A couple patches have been previously proposed to change the ordering
of startup and shutdown processing, or to put a mutex around
everything related to the tap/macvtap device name usage, but in the
end no matter what you do there will still be possible holes, because
the device could be deleted outside libvirt's control (for example,
regular tap devices are automatically deleted when the qemu process
terminates, and that isn't always initiated by libvirt but could
instead happen completely asynchronously - libvirt then has no control
over the ordering of shutdown operations, and no opportunity to
protect it with a mutex.)

But this only happens if a new device is created at the same time as
one is being deleted. We can effectively eliminate the chance of this
happening if we end the practice of always looking for the lowest
numbered available device name, and instead just keep an integer that
is incremented each time we need a new device name. At some point it
will need to wrap back around to 0 (in order to avoid the IFNAMSIZ 15
character limit if nothing else), and we can't guarantee that the new
name really will be the *least* recently used name, but "math"
suggests that it will be *much* less common that we'll try to re-use
the *most* recently used name.

This patch implements such a counter for macvtap/macvlan, replacing
the existing, and much more complicated, "ID reservation" system. The
counter is set according to whatever macvtap/macvlan devices are
already in use by guests when libvirtd is started, incremented each
time a new device name is needed, and wraps back to 0 when either
INT_MAX is reached, or when the resulting device name would be longer
than IFNAMSIZ-1 characters (which actually is what happens when the
template for the device name is "maccvtap%d"). The result is that no
macvtap name will be re-used until the host has created (and possibly
destroyed) 99,999,999 devices.

Signed-off-by: Laine Stump <laine redhat com>
  src/libvirt_private.syms    |   1 -
  src/libxl/libxl_driver.c    |   2 +-
  src/lxc/lxc_process.c       |   2 +-
  src/qemu/qemu_process.c     |   2 +-
  src/util/virnetdevmacvlan.c | 402 +++++++++++++-----------------------
  src/util/virnetdevmacvlan.h |   6 +-
  6 files changed, 145 insertions(+), 270 deletions(-)

diff --git a/src/util/virnetdevmacvlan.c b/src/util/virnetdevmacvlan.c
index dcea93a5fe..dc4db2c844 100644
--- a/src/util/virnetdevmacvlan.c
+++ b/src/util/virnetdevmacvlan.c

+static int
+virNetDevMacVLanGenerateName(char **ifname, unsigned int flags)
-    unsigned int id;
-    unsigned int flags = 0;
-    const char *idstr = NULL;
-    if (virNetDevMacVLanInitialize() < 0)
-       return -1;
+    const char *prefix;
+    const char *iftemplate;
+    int *lastID;
+    int id;
+    double maxIDd;
+    int maxID = INT_MAX;
+    int attempts = 0;
-        idstr = name + strlen(VIR_NET_GENERATED_MACVTAP_PREFIX);
-        idstr = name + strlen(VIR_NET_GENERATED_MACVLAN_PREFIX);
+        iftemplate = VIR_NET_GENERATED_MACVTAP_PREFIX "%d";
+        lastID = &virNetDevMacVTapLastID;
      } else {
-        return -2;
+        iftemplate = VIR_NET_GENERATED_MACVLAN_PREFIX "%d";
+        lastID = &virNetDevMacVLanLastID;
  -    if (virStrToLong_ui(idstr, NULL, 10, &id) < 0) {
-        virReportError(VIR_ERR_INTERNAL_ERROR,
-                       _("couldn't get id value from macvtap device name %s"),
-                       name);
-        return -1;
-    }
-    return virNetDevMacVLanReserveID(id, flags, quietFail, false);
+    maxIDd = pow(10, IFNAMSIZ - 1 - strlen(prefix));
+    if (maxIDd <= (double)INT_MAX)
+        maxID = (int)maxIDd;

pow() requires -lm. We need this to be squashed in:

Dan had said yesterday in IRC that we already link in libm, and it's been building correctly. Are there other targets where that isn't the case, and I'm just lucky?

diff --git i/meson.build w/meson.build
index dabd4196e6..81668a6681 100644
--- i/meson.build
+++ w/meson.build
@@ -1176,6 +1176,9 @@ endif
 libxml_version = '2.9.1'
 libxml_dep = dependency('libxml-2.0', version: '>=' + libxml_version)

+cc = meson.get_compiler('c')
+m_dep = cc.find_library('m', required : false)
 use_macvtap = false
 if not get_option('macvtap').disabled()
   if (cc.has_header_symbol('linux/if_link.h', 'MACVLAN_MODE_BRIDGE') and
diff --git i/src/util/meson.build w/src/util/meson.build
index a7017f459f..f7092cc3f1 100644
--- i/src/util/meson.build
+++ w/src/util/meson.build
@@ -188,6 +188,7 @@ virt_util_lib = static_library(
+    m_dep,

NOTE: Doesn't come from my head.


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