> From: Ladi Prosek [mailto:lpro...@redhat.com] > Sent: Tuesday, 7 November 2017 00:11 > > On Mon, Nov 6, 2017 at 7:41 PM, Andrew Baumann > <andrew.baum...@microsoft.com> wrote: > > Hi, > > > > I just noticed this thread, and the problem sounded very familiar... > > > >> From: Ladi Prosek > >> Sent: Monday, 6 November 2017 07:16 > > [...] > >> FS base passes all the checks in windbg_on_load() as the guest kernel > >> loads and it returns true. QEMU then sends some data over the pipe. > >> Windbg doesn't print anything, it's still showing: > >> > >> Opened \\.\pipe\win7_dbg > >> Waiting to reconnect... > >> > >> Is this expected? In regular remote kernel debugging, windbg produces > >> a bunch of output about the target state when it attaches. > > > > Just a guess, but I suspect the problem you're seeing may be due to the > combination of windbg's tight timing requirements for the serial debug > protocol and qemu's polling-based implementation of named pipe IO. I ran > into this a year or two ago when bringing up raspberry pi... I could never > reliably connect windbg to the emulated serial port using a named pipe. I > believe the problem is that the named pipe driver relies on receiving input > through win_chr_pipe_poll(), which is called too infrequently to satisfy > windbg's needs (it has a rather short timeout before it gives up and > retransmits). The correct fix is to use blocking IO for the named pipes inside > qemu, but I know from experience that win32 is awful in this respect -- qemu > wants a single unified blocking call to wait for IO on any combination of > named pipes, sockets, events, etc. and Windows appears to lack this, at least > at the level of poll/select. > > Thanks, I don't usually set up windbg remote debugging over named > pipes so this indeed sounded plausible. > > My Windows runs in a QEMU/KVM VM so to eliminate the pipe I have set > up a virtual COM port like so: > > -serial tcp:127.0.0.1:8889,server,nowait > > and the nested QEMU ran with: > > -windbg tcp:10.0.2.2:8889 > > where 10.0.2.2 is the address assigned to the host with user > networking. Windbg then used the default COM1 to connect to the > target. I routinely use COMx for remote debugging between VMs with a > TCP connection on the host side acting as a null-modem cable and never > had any timing related issues. > > However, the symptoms were exactly the same with this set up.
Ok, then I agree this is probably not the same issue. Andrew
