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Diffstat (limited to 'results/classifier/105/socket/778032')
| -rw-r--r-- | results/classifier/105/socket/778032 | 108 |
1 files changed, 108 insertions, 0 deletions
diff --git a/results/classifier/105/socket/778032 b/results/classifier/105/socket/778032 new file mode 100644 index 000000000..f0a262c7e --- /dev/null +++ b/results/classifier/105/socket/778032 @@ -0,0 +1,108 @@ +instruction: 0.958 +socket: 0.943 +semantic: 0.941 +assembly: 0.940 +other: 0.928 +device: 0.925 +network: 0.920 +graphic: 0.920 +boot: 0.913 +vnc: 0.892 +KVM: 0.885 +mistranslation: 0.791 + +qemu spinning on serial port writes + +As originally found at http://<email address hidden>/msg08745.html from 3 years ago! + +Basically qemu seizes up in the event that the file descriptor for its emulated serial port has a full buffer, i.e. write() returns EAGAIN. For me, this happened when the serial port was being directed through a UNIX socket, with a default-sized 4KB buffer. Just the normal output from a Linux kernel boot caused it to seize up, and stop the main emulation / select loop. + +My suggestion is to remove the detection of EAGAIN in qemu-char.c:521, so that if the buffer is full, KVM discards the byte(s) it was trying to write. This is a surely better outcome than the process spinning forever. + +I will submit a separate patch to control the buffer sizes when creating UNIX sockets, which will help allow slow-reading processes to tune things so that they don't miss any output. + +Additionally, in the context of a hosted environment, if the -serial option is used, this could be a small security issue. An untrusted user of a guest system, knowing their serial output is going via a small buffer, could spew output to their /dev/ttyS0 at a rate fast enough to trigger this bug and eat a CPU core on the host. + +To quote David S. Ahern's original bug report (mine was the same, only with the latest version from git, so line numbers may have changed - my suggested fix above is accurate though): + +I am trying to redirect a guest's boot output through the host's serial +port. Shortly after launching qemu, the main thread is spinning on: + +write(9, "0", 1) = -1 EAGAIN (Resource temporarily unavailable) + +fd 9 is the serial port, ttyS0. + + +The backtrace for the thread is: + +#0 0x00002ac3433f8c0b in write () from /lib64/libpthread.so.0 +#1 0x0000000000475df9 in send_all (fd=9, buf=<value optimized out>, +len1=1) at qemu-char.c:477 +#2 0x000000000043a102 in serial_xmit (opaque=<value optimized out>) at +/root/kvm-81/qemu/hw/serial.c:311 +#3 0x000000000043a591 in serial_ioport_write (opaque=0x14971790, +addr=<value optimized out>, val=48) + at /root/kvm-81/qemu/hw/serial.c:366 +#4 0x00000000410eeedc in ?? () +#5 0x0000000000129000 in ?? () +#6 0x0000000014821fa0 in ?? () +#7 0x0000000000000007 in ?? () +#8 0x00000000004a54c5 in tlb_set_page_exec (env=0x10ab4, +vaddr=46912496956816, paddr=1, prot=-1, mmu_idx=0, is_softmmu=1) + at /root/kvm-81/qemu/exec.c:388 +#9 0x0000000000512f3b in tlb_fill (addr=345446292, is_write=1, +mmu_idx=-1, retaddr=0x0) + at /root/kvm-81/qemu/target-i386/op_helper.c:4690 +#10 0x00000000004a6bd2 in __ldb_cmmu (addr=9, mmu_idx=0) at +/root/kvm-81/qemu/softmmu_template.h:135 +#11 0x00000000004a879b in cpu_x86_exec (env1=<value optimized out>) at +/root/kvm-81/qemu/cpu-exec.c:628 +#12 0x000000000040ba29 in main (argc=12, argv=0x7fff67f7a398) at +/root/kvm-81/qemu/vl.c:3816 + +send_all() invokes unix_write() which by design is not breaking out on +EAGAIN. + +The following command is enough to show the problem: + +qemu-system-x86_64 -m 256 -smp 1 -no-kvm \ + -drivefile=/dev/cciss/c0d0,if=scsi,cache=off,boot=on \ + -vnc :1 -serial /dev/ttyS0 + + +The guest is running RHEL3 with the parameter 'console=ttyS0' added to +grub.conf; the problem appears to be with qemu, so I would expect it to +show with any linux guest. This particular host is running RHEL5.2 with +kvm-81, but I have also seen the problem with Fedora-9 as the host OS. + +Yes, the serial port of the server is connected to another system via a +null modem. If I change the serial argument to '-serial udp::4555' and +use 'nc -u -l localhost 4555 > /dev/ttyS0' I see the guest's boot +output show up on the second system as expected. I'd prefer to be able +to use the serial port connection directly without nc as a proxy. +Suggestions? + +I have a similar problem, and the problem goes away if I remove the "-serial pty" device from my command line: + +/home/mrhines/qemu/x86_64-softmmu/qemu-system-x86_64 /kvm_repo/cb/vmbase -serial pty + +Such a simple command line, but QEMU seizes up and slows to a crawl and the host CPU is spinning at 100% + +Which version of QEMU have you been using here? Can you still reproduce this problem with the latest version of QEMU (currently version 2.9)? + +I'm confident this has been solved a while ago. When this bug was reported, the code was indeed broken wrt EAGAIN handling. + +The chardev code has long since been re-written though, and the send_all method replaced by io_channel_send_all() which will handle EAGAIN by returning instead of spinning in a loop. + +Specifically this commit changed the code to stop spinning in send_all() on EAGAIN + +commit 23673ca740e0eda66901ca801a5a901df378b063 +Author: Anthony Liguori <email address hidden> +Date: Tue Mar 5 23:21:23 2013 +0530 + + qemu-char: add watch support + + This allows a front-end to request for a callback when the backend + is writable again. + + |