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

[Charles Duffy <cduffy-0XdUWXLQalXR7s880joybQ@public.gmane.org>]

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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