RE: [Xen-devel] faster windows debugging design

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

> On Tue, Oct 19, 2010 at 11:02:26PM +1100, James Harper wrote:
> > Here's my plan so far...
> >
> > Windows debugging is done via the serial port (or 1394 or USB). The
> > current way of debugging a windows DomU is to connect 'xm console'
to a
> > tcp port in domU, and then on a separate machine (virtual or
physical)
> > connect that tcp port to a virtual serial port or a named pipe. This
> > tends to suffer from fairly poor performance though as anyone who's
used
> > it will confirm.
> >
> > My approach is to write a new kd module - kdxen.dll - under windows
> > which is loaded by specifying /debugport=xen in boot.ini. This sets
up
> > an alternate communication channel with Xen that doesn't involve a
> > vmexit every time a byte is read or written via the serial port.
> > xen_platform.c is modified to control the other end of that
> > communication channel (I'm using the same 0x10-1F ioport range to
set up
> > the channel at the moment, otherwise it could possibly go in a
separate
> > module altogether. Needs to be a fixed port though.). I/O is done
via
> > the backend of the serial port interface
(qemu_chr_write(serial_hds[0],
> > ...) which means that no change is required past dom0. Additionally,
the
> > debugger does a lot of busywaiting for I/O (no interrupts are used)
> > which can be deferred to usleeps in qemu which should reduce system
load
> > a bit.
> 
> Would it make sense to provide a "failsafe" mechanism for debugging?
Say
> you need to debug this driver, and want to use the old mechanism. Or
> is that not a problem since the old mechanism is rock solid and won't
> be impared?

If you specify /debugport=xen then my kdxen.dll module will be loaded,
will signal to xen that high speed debug is in progress, and disable the
com port. If you leave the /debugport option as the default =comX then
kdcom.dll will be loaded and things will be the same as they always
were.

> >
> > So the changes to qemu that I can see so far are:
> > . additional code to control the setup and teardown of communication
> > rings (tx and rx ring)
> > . a way to divert received I/O from the serial port to my faster
> > interface so that the serial port doesn't eat data
> >
> > Any comments?
> >
> > The other possible approach is to emulate a 1394 interface just
enough
> > to make windows sufficiently happy to use it as a debug interface.
This
> > is quite a bit more complex though, qemu would need to implement a
new
> > PCI device, and also deal with the fact that RDMA is not actually
> > available end-to-end (eg over tcp) so qemu would need knowledge of
the
> > windbg protocol to convert to a serial stream of data compatible
with
> > the serial windbg protocol. I'm not completely sure about any of
that
> > though.
> 
> USB debugging? There is a USB device already there (in QEMU), and both
> Linux and Windows have the USB debug port stack implemented quite
robustly.
> (Thought I am not sure how well they work past a S3 sleep/resume).

USB debugging needs a EHCI (2.0) interface that supports USB debugging
(qemu usb is 1.1), and is only supported under Vista and later, so it's
less attractive.

James

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