kvm-36 losing (tap-based) network connectivity mid-session

kvm-36 is repeatedly timing out partway through a long network transfor and failing to resume. Both host and guest are 64-bit. This has happened with a number of prior releases as well; it does not appear to be a recently-introduced issue.

My command line looks like the following:

$ qemu-system-x86_64 -kernel kernel-x86_64-2.6.20-gentoo-r8 -initrd 
initramfs.igz -hda hda -hdb hdb -append "panic=30 console=ttyS0 
action=install" -nographic -net nic -net 
'tap,ifname=tap0,script=./tap0-start'

/dev/kvm is quite certainly being used, as KVM immediately segfaults on startup without it (or in the presence of the -no-kvm option).

dmesg on the host shows a great number of "rtc: lost some interrupts at 1024Hz" messages.

The end of a typical session looks much like the following:

====
79000K .......... .......... .......... .......... .......... 4% 5.43M
4:33
79050K .......... .......... .......... .......... .......... 4% 6.10M
4:03
79100K .......... .......... .......... .......... .......... 4% 8.14M
3:02
79150K ...... 4% 5.96M
4:09

02:45:23 (2.06 MB/s) - Read error at byte 81055847/1639127012 (Connection timed out). Retrying.

--02:45:23--
http://10.0.0.30/gentoo-installer/deployed_server-stage4-amd64-2007.0.tar.bz2
(try: 2) => `/mnt/gentoo/deployed_server-stage4-amd64-2007.0.tar.bz2'
Connecting to 10.0.0.30:80... Hangcheck: hangcheck value past margin!
failed: Connection timed out.
Retrying.

--02:45:53--
http://10.0.0.30/gentoo-installer/deployed_server-stage4-amd64-2007.0.tar.bz2
(try: 3) => `/mnt/gentoo/deployed_server-stage4-amd64-2007.0.tar.bz2'
Connecting to 10.0.0.30:80... failed: Connection timed out. Retrying.

--02:46:23--
http://10.0.0.30/gentoo-installer/deployed_server-stage4-amd64-2007.0.tar.bz2
(try: 4) => `/mnt/gentoo/deployed_server-stage4-amd64-2007.0.tar.bz2'
====

Network access works, and then suddenly doesn't. I am more reliably able to reproduce this issue using -net tap than -net user.

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happened

`/mnt/gentoo/deployed_server-stage4-amd64-2007.0.tar.bz2'

`/mnt/gentoo/deployed_server-stage4-amd64-2007.0.tar.bz2'

`/mnt/gentoo/deployed_server-stage4-amd64-2007.0.tar.bz2'

able

Can you try it without rtc (either use -no-rtc, I'm not sure we have this flag or
just rm /dev/rtc before you run the vm).
What's your host HZ configuration?
If the above doesn't work, can you repeat the test with -no-kvm?

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Tried it (and -tdf) without effect.

CONFIG_HZ=250

-no-kvm segfaults immediately on startup.

===
Starting program: /usr/bin/kvm -no-kvm -kernel kernel-x86_64-2.6.20-gentoo-r8 -hda grubdisk.img
[Thread debugging using libthread_db enabled]
[New Thread 47987424286272 (LWP 13655)]
Could not configure '/dev/rtc' to have a 1024 Hz timer. This is not a fatal error, but for better emulation accuracy either use a 2.6 host Linux kernel or
type 'echo 1024 > /proc/sys/dev/rtc/max-user-freq' as root.

Program received signal SIGSEGV, Segmentation fault. [Switching to Thread 47987424286272 (LWP 13655)] 0x0000000000000000 in ?? ()
(gdb) bt

#0 0x0000000000000000 in ?? ()
#1 0x0000000000481e8b in cpu_x86_exec (env1=<value optimized out>) at /usr/src/debug/app-emulation/kvm-36/kvm-36/qemu/cpu-exec.c:750 #2 0x0000000000409a6d in main_loop () at /usr/src/debug/app-emulation/kvm-36/kvm-36/qemu/vl.c:6311
#3 0x000000000040de7a in main (argc=0, argv=0x7fffba1c59c8) at /usr/src/debug/app-emulation/kvm-36/kvm-36/qemu/vl.c:7774
#4 0x00002ba4f12ea5e6 in __libc_start_main () from /usr/local/lib/libc.so.6 #5 0x0000000000405689 in _start ()
===

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Can you check with the other emulated nic? (-nic model=...)

error compiling committee.c: too many arguments to function


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Happens with both ne2k_pci and rtl8139; I haven't tried with others.

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As of kvm-43, this is no longer happening. I haven't tested intermediate versions to locate the exact place where the issue ceased.

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