Re: [Xen-devel] Re: xl vs. xm, possible bug in xl

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 9 Sep 2011, Sven Köhler wrote:
> Am 09.09.2011 13:27, schrieb Stefano Stabellini:
> >> Any clue, that the second problem is about?
> >>> # xl create /etc/xen/xen-sk1
> >>> Parsing config file /etc/xen/xen-sk1
> >>> libxl: error: libxl_device.c:476:libxl__wait_for_device_model Device 
> >>> Model not ready
> >>> xl: fatal error: libxl_create.c:535, rc=-1: 
> >>> libxl__confirm_device_model_startup
> > 
> > That means that qemu failed to start. Could you please cat
> > /var/log/xen/qemu-dm-domainname.log?
> 
> There is no such file (my domain config lacks a name="something" line).
> However qemu-dm-test.log does exist and is of recent date, and it says
> that qemu cannot be started. This is very plausible, since qemu is not
> even installed. This machine is supposed to start paravirt guests only.
> And xen has been compiled without support for hvm guests. (Not sure
> right now, what the gentoo people do to disable support support for hvm
> guests).
> 
> Does my config file for the domain (see one of my previous emails in
> this thread) indicate, that the machine is a hvm domain? How can I tell
> xl that this a paravirt domain, and qemu is not needed and should not be
> used?

I think I have found the issue: if blktap2 is not enabled xl is going to
start qemu (to provide a disk backend) even if it is not actually needed
because the user wants to use blkback.

We have a patch upstream to fix this issue but it hasn't been backported
to 4.1:



# HG changeset patch
# User Ian Campbell <ian.campbell@xxxxxxxxxx>
# Date 1300213187 0
# Node ID d4ca456c0c25c3c3daedc216c657296f2895482a
# Parent  3caed2112c65791855e8bf0fd34c15e3160bbc78
libxl: do not start a xenpv qemu solely for tap devices if blktap is available

qemu is used as a fallback for DISK_BACKEND_TAP if no blktap is
available but if blktap is available, or for DISK_BACKEND_PHY, we
don't need a qemu process.

Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
Acked-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Committed-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>

diff -r 3caed2112c65 -r d4ca456c0c25 tools/libxl/libxl_dm.c
--- a/tools/libxl/libxl_dm.c    Tue Mar 15 10:14:27 2011 +0000
+++ b/tools/libxl/libxl_dm.c    Tue Mar 15 18:19:47 2011 +0000
@@ -828,8 +828,29 @@ int libxl__need_xenpv_qemu(libxl_ctx *ct
         goto out;
     }
 
-    if (nr_disks > 0 && !libxl__blktap_enabled(&gc))
-        ret = 1;
+    if (nr_disks > 0) {
+        int blktap_enabled = -1;
+        for (i = 0; i < nr_disks; i++) {
+            switch (disks[i].backend) {
+            case DISK_BACKEND_TAP:
+                if (blktap_enabled == -1)
+                    blktap_enabled = libxl__blktap_enabled(&gc);
+                if (!blktap_enabled) {
+                    ret = 1;
+                    goto out;
+                }
+                break;
+
+            case DISK_BACKEND_QDISK:
+                ret = 1;
+                goto out;
+
+            case DISK_BACKEND_PHY:
+            case DISK_BACKEND_UNKNOWN:
+                break;
+            }
+        }
+    }
 
 out:
     LIbxl__free_all(&gc);

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