After some debugging and debugging, with a help Hollis Blanchard on #kvm@freenode, I discovered that kvm (or, rather, qemu) does not work correctly with serial ports, at least on linux. One problem report has already here, author Cc'd -- see e.g. http://marc.info/?l=kvm&m=122995568009533&w=2 . Here's what's going on. When opening a host's port, kvm resets the status lines, doing this: ioctl(13, TIOCMGET, [TIOCM_DTR|TIOCM_RTS|TIOCM_CTS|TIOCM_DSR|0x4000]) ioctl(13, TIOCMSET, [TIOCM_DTR|TIOCM_RTS]) which results in the following set ioctl(13, TIOCMGET, [TIOCM_DTR|TIOCM_RTS|TIOCM_CTS|TIOCM_DSR]) Note the difference between the default set and new one: the missing bit, 0x4000, which is unknown to strace(1) but is defined as TIOCM_OUT2 in linux headers. After that change (resetting the TIOCM_OUT2 bit), no writes to the serial port works anymore, they're all gets accepted by host kernel and are buffered in the kernel. After some time, when the kernel buffer fills up, and since the port (on host side) is opened in non-blocking mode, the writes starts returning EAGAIN, and kvm process starts endless loop, which were seen by David. Here's the trivial program to demonstrate the idea: ---- cut --- #include <sys/types.h> #include <unistd.h> #include <fcntl.h> #include <sys/ioctl.h> #include <termios.h> int main(int argc, char **argv) { fd = open("/dev/ttyS0", O_RDWR|O_NONBLOCK); fcntl(fd, F_SETFL, O_RDWR); x = TIOCM_DTR|TIOCM_RTS|TIOCM_CTS|TIOCM_DSR // |0x4000 ; ioctl(fd, TIOCMSET, &x); ioctl(fd, TIOCMGET, &x); write(fd, "at\r", 3); read(fd, buf, 20); close(fd); return 0; } --- cut --- Run it under strace while a dialup modem is connected to /dev/ttyS0 (i used this way for testing). It will stuck at read, and nothing will be written, even when write() will happily return 3. Un-comment the |0x4000 thing, and it will work. I'm not sure what should be done with this, and how much this is linux-specific. But it is obvious that bit (TIOCM_OUT2) should be left in-place (after which the thing works), at least on linux. Note that this bit is NOT shown in /proc/tty/driver/serial file (which shows other bits). Note also that this file (/proc/tty/driver/serial) helps to see if any write were performed: compare the counters. In 'tx' there's number of bytes actually sent to device, as opposed to accepted by the kernel. When you write something to /dev/ttyS0, that number increases, IF that something actually reached the device. Thanks. /mjt -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
