In debugging a virtbench bug on KVM, I seemed to uncover some weird 
behaviors with gettimeofday() and KVM with newer kernels.

The attached program will run a tight loop of code a certain number of 
times and measure it's duration with gettimeofday().  With a stock 
ubuntu 2.6.20-15 kernel, I see pretty reasonable behavior in the guest:

./loop 0
0
./loop 100
0.5
./loop 10000
0.150
./loop 1000000
0.15954

Note the format is <secs>.<usecs> so it's behaving quite well.  Now, 
with a 2.6.22-rc4 kernel, using the same kernel config (saying N to all 
of the new features), we get:

./loop 0
0.16
./loop 100
0.0
./loop 10000
0.0
./loop 1000000
0.20008

It's pretty erratic.  If I run if a few times at any rate, sometimes I 
get 0.0 and sometimes I get a more sane result.

I've attached the .config for 2.6.22-rc4 and loop.c.  I haven't yet 
starting bisecting KVM versions and kernel versions.  I know the time 
stuff has changed a lot in 2.6.21,22 so I'm wondering if someone has an 
idea of what the problem could be.  The same problem occurs with AMD and 
Intel.

Regards,

Anthony Liguori