Xen devel - Oct 2006 - [PATCH][RFC] addition of loglevel for printf in Xen HV

In the current implementation, the logging in the Hypervisor is 
basically at two levels.  The developer can print "always" or
"verbose".
  The always print is when the developer uses "printk/printf" in the 
hypervisor.  The "verbose" is when the developer uses DPRINTK, since
the
DPRINTK macro is only defined when the "verbose" flag is set.

Now the problem we are having, is that when we send out a distribution, 
we want enough information to be printed if one of our customers has a 
problem.  But we also don''t want to have so much printed that it
affects
the performance of the system.  Or worst yet, if the system has a serial 
console attached, one of the DomU''s could in essence cause a DoS attack
on the whole system by causing the DomU to do certain errors that causes 
a thousand prints to the serial console.  These prints are done with 
interrupts disabled, so the system suffers large latencies for each print.

Since it would be helpful to have prints for different environments, 
I''ve introduced with this patch a Linux like loglevel from 0-7. When
the
user sets the loglevel to 0, only emergency prints are preformed, and 
when 7 is selected, all prints are done.

Here''s the list of levels that I have added:

#define XEN_EMERG   "<0>"
#define XEN_ALERT   "<1>"
#define XEN_CRIT    "<2>"
#define XEN_ERR     "<3>"
#define XEN_WARNING "<4>"
#define XEN_GUEST   "<5>"
#define XEN_INFO    "<6>"
#define XEN_DEBUG   "<7>"

Basically, these correspond to their Linux equivalents. The one 
difference, is that the KERN_NOTICE was replaced by XEN_GUEST.  The 
XEN_GUEST is the level that a guest can "at will" cause this print. 
So
a distribution can set the print log level to 4 (XEN_WARNING) and all 
prints that are of errors and can be used for debugging are now 
available, without the prints that allow a Guest to cause a DoS.

I also added the keyhandlers ''0'' to ''7'' to
allow the user to change the
loglevel on a running system to the corresponding key. This is similar 
to the way Linux uses sysrq to change its log levels.

Two things that are not implemented with this patch but should be, and 
thus the RFC in the subject.

1. How to default the log level on build. Should the verbose now be a 
number, and not a config?  i.e. VERBOSE=4

2. How the user in Linux can change the log level (from dom0).  Should 
we add a hypercall or some other method to allow the user to change the 
logging?

Also, I made a pass though to assign all DPRINTK a log level.  I may not 
have done this correctly.  But a loglevel of 7 is now the same a 
VERBOSE=y  and a loglevel of 0 is almost the same of a VERBOSE=n. There 
was a couple of places where XEN_EMERG was used.

This patch really does need to go in.  Whether in its current form, or 
modified greatly.  The ability to give the developer a level of printing 
is a great asset.  That is why Linux has this ability.

-- Steve


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Forgot to add a sign off.

-- Steve

Signed-off-by: Steven Rostedt <srostedt@redhat.com>

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