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Re: [PATCH] qemu: configurable VNC port boundaries for domains

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On 05/22/2012 09:00 AM, Dave Allan wrote:
> On Tue, May 22, 2012 at 04:10:03PM +0200, Martin Kletzander wrote:
>> The defines QEMU_VNC_PORT_MIN and QEMU_VNC_PORT_MAX were used to find
>> free port when starting domains. As this was hardcoded to the same
>> ports as default VNC servers, there were races with these other
>> programs. This patch includes the possibility to change the default
>> starting port as well as the maximum port in qemu config file.
> 
> Hi Martin,
> 
> Two design comments:
> 
> 1) https://bugzilla.redhat.com/show_bug.cgi?id=782814 requests that
> the default port be changed to avoid conflicts, which seems reasonable
> to me.

If we choose better defaults for new installations, we still need to
worry about preserving existing ranges when upgrading old installations.
 This may need some coordination with the spec file doing some %post
magic to add in vnc_port_min with a value other than 5900 to qemu.conf
on new installations, but leaving it unspecified when doing an upgrade.

> 
> 2) I agree with the config option since most applications on the
> system will want the system defaults.  However, IMO in this case an
> application writer should be given the option in the XML to override
> the system default.

Agreed - I think we need both solutions - qemu.conf to specify the
default range, and per-domain XML to specify an override (does the XML
need to specify a range, or just a single port?).


>> +++ b/src/qemu/qemu_command.h
>> @@ -37,6 +37,9 @@
>>  # define QEMU_VIRTIO_SERIAL_PREFIX "virtio-serial"
>>  # define QEMU_FSDEV_HOST_PREFIX "fsdev-"
>>
>> +/* These are only defaults, they can be changed now in qemu.conf and
>> + * explicitely specified port is checked against these two (makes

s/explicitely/explicitly/


>>
>> +    p = virConfGetValue (conf, "vnc_port_min");
>> +    CHECK_TYPE ("vnc_port_min", VIR_CONF_LONG);
>> +    if (p) {
>> +        if (p->l < QEMU_VNC_PORT_MIN) {
>> +            /* if the port is too low, we can't get the display name
>> +             * tell to vnc (usually subtract 5900, e.g. localhost:1
>> +             * for port 5901) */
>> +            qemuReportError(VIR_ERR_INTERNAL_ERROR,
>> +                            _("%s: vnc_port_min: port must be greater than or equal to %d"),
>> +                            filename, QEMU_VNC_PORT_MIN);

You ought to mention this restriction in qemu.conf.


>> +    p = virConfGetValue (conf, "vnc_port_max");
>> +    CHECK_TYPE ("vnc_port_max", VIR_CONF_LONG);
>> +    if (p) {
>> +        if (p->l > QEMU_VNC_PORT_MAX ||
>> +            p->l <= driver->vncPortMin) {

Off-by-one.  You want to allow min==max for specifying a range of
exactly 1 port, useful if you are only running one guest and want to
guarantee its port.  You need p->l < driver->vncPortMin, not p->l <=
driver->vncPortMin.


>> +++ b/src/qemu/qemu_conf.h
>> @@ -89,6 +89,8 @@ struct qemud_driver {
>>      unsigned int vncSASL : 1;
>>      char *vncTLSx509certdir;
>>      char *vncListen;
>> +    long vncPortMin;
>> +    long vncPortMax;

long?  We know it's only 16 bits; short would do, int might be easier to
work with, but long is overkill.

-- 
Eric Blake   eblake@xxxxxxxxxx    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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