Re: [Xen-devel] libvirt: [PATCH] libxl: Check for control_d string to decide about dom0

[Stefan Bader <stefan.bader@xxxxxxxxxxxxx>]

On 12.03.2014 13:08, Ian Campbell wrote:
> On Wed, 2014-03-12 at 13:03 +0100, Stefan Bader wrote:
>> I have been looking into a bug report (see BugLink) which reported
>> libvirt to fail starting inside a Xen guest. Upon further investigation
>> I found that some tools that help monitoring Xen guests will mount
>> xenfs to /proc/xen. This will create a capabilities files there even
>> if the guest is not dom0. However it will return nothing when reading
>> from it.
> 
> This seems consistent with the xencommons initscript which does:
>         # run this script only in dom0:
>         # no capabilities file in xenlinux domU kernel
>         # empty capabilities file in pv_ops domU kernel
>         if test -f /proc/xen/capabilities && \
>            ! grep -q "control_d" /proc/xen/capabilities ; then
>                 exit 0
>         fi
> 
>>
>> Ian, just to sanity check myself. I looked at the xenfs code and to
>> me there only seem to be those two outcomes (either "control_d" for
>> running in dom0 or notrhing if not).
>>
>> With the following patch applied, libvirt starts up correctly in
>> the normal guests (with xenfs mounted) without initializing libxl.
>> And also in dom0 where it still enables the libxl driver (if the
>> xl toolstack is selected).
>>
>> -Stefan
>>
>> From f11949caca6dfe1a802472a2a6d4fe760115ccc6 Mon Sep 17 00:00:00 2001
>> From: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
>> Date: Wed, 12 Mar 2014 11:37:16 +0100
>> Subject: [PATCH] libxl: Check for control_d string to decide about dom0
>>
>> As soon as any guest mounts xenfs to /proc/xen, there is a capabilities
>> file in that directory. However it returns nothing when reading from it.
>> Change the test to actually check the contents of the file.
>>
>> BugLink: http://bugs.launchpad.net/bugs/1248025
>>
>> Signed-off-by: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
>> ---
>>  src/libxl/libxl_driver.c |   14 ++++++++++++--
>>  1 file changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/src/libxl/libxl_driver.c b/src/libxl/libxl_driver.c
>> index 65d80a2..844e828 100644
>> --- a/src/libxl/libxl_driver.c
>> +++ b/src/libxl/libxl_driver.c
>> @@ -944,6 +944,7 @@ libxlDriverShouldLoad(bool privileged)
>>      bool ret = false;
>>      virCommandPtr cmd;
>>      int status;
>> +    char *output = NULL;
>>  
>>      /* Don't load if non-root */
>>      if (!privileged) {
>> @@ -951,8 +952,17 @@ libxlDriverShouldLoad(bool privileged)
>>          return ret;
>>      }
>>  
>> -    /* Don't load if not running on a Xen control domain (dom0) */
>> -    if (!virFileExists("/proc/xen/capabilities")) {
>> +    /*
>> +     * Don't load if not running on a Xen control domain (dom0). It is not
>> +     * sufficient to check for the file to exist as any guest can mount
>> +     * xenfs to /proc/xen.
>> +     */
>> +    status = virFileReadAll("/proc/xen/capabilities", 10. &output);
> 
> Is this "." supposed to be a ","?

Darn, I thought I had fixed that. Unfortunately in the wrong place. Yes, that
should be a ",". Sorry

-Stefan
> 
>> +    if (status >= 0) {
>> +        status = strncmp(output, "control_d", 9);
>> +    }
>> +    VIR_FREE(output);
>> +    if (status) {
>>          VIR_INFO("No Xen capabilities detected, probably not running "
>>                   "in a Xen Dom0.  Disabling libxenlight driver");
>>  
> 
>

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