RE: [Xen-devel] debug logging in windows

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> 
> > Is there something similar implemented via hvm/qemu that can log via
hvm
> > log files? That would make life easier. If not, I'll implement it.
> There isn't at the moment, at least not in the xenbits repositories.
> There is one in the XenServer version of qemu, though.  I've attached
> the relevant patch.  If you apply this and rebuild qemu then you
> should be able to send stuff to the qemu logfile by writing bytes to
> port 0x12.  Note that this is rate limited, though, and if you hit the
> limiter the domain will be quite severely punished.
> 
> I've checked that this applies against qemu-xen-unstable.git, and that
> the resulting tree at least builds, but I've not actually checked that
> it works.  Upstream qemu is pretty similar to XenServer qemu at the
> moment, though, so it should be okay.
> 

I'd love to see this in 3.3.1... what are the chances?

The only change I'd like to make is one that I did in my own version
which is to keep separate log buffers per io port (with the intent that
each DomU CPU writes to a different port), as I was finding the log
messages from different CPU's writing at the same time causing garbled
messages. Synchronising access to the io port between different drivers
in windows would require a heap of overhead.

By making sure in the DomU that each CPU uses port (0x10 + (cpu# &
0x0f)) each CPU only has to disable interrupts on the local CPU while
writing the log message. Obviously with #cpu's > 16 you will get some
merged log messages, but this would be less of a problem than the mess I
was seeing when logging at a high rate.

Basically the change is to use the full range of 16 ports, and declare
log_buffer[16][4096] and use the port offset from 0x10 as the first
index into the array, and obviously index all the other variables that
manage the buffer. I also prefix each message with the CPU number (eg
'06: '). The throttling would remain global.

Having this in 3.3.1 would make it heaps easier to get debug info from
people having problems with the GPLPV drivers!

I can put my changes in and submit a patch if the above sounds
reasonable...

Thanks

James


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