high load with win7 & usb tablet

[Michael Tokarev <mjt@tls.msk.ru>]

Hello.

I noticed that window7, unlike windowsXP as I played with
previously, poses quite high load on host when idle.  On
my machine, host cpu usage is about 25% when one single-cpu
win7 guest is running with "-usbdevice tablet" option.
Here's a typical kvm_stats when such a guest is idling:

 efer_reload                  0       0
 exits                   991832     695
 fpu_reload                3786       0
 halt_exits                7759      67
 halt_wakeup               7717      67
 host_state_reload       613116     495
 hypercalls                   0       0
 insn_emulation          873244     488
 insn_emulation_fail          0       0
 invlpg                       0       0
 io_exits                582925     409
 irq_exits                30188       8
 irq_injections           41093       0
 irq_window                   0       0
 largepages                 288       0
 mmio_exits               16718       0
 mmu_cache_miss             217       0
 mmu_flooded                  0       0
 mmu_pde_zapped               0       0
 mmu_pte_updated              0       0
 mmu_pte_write                0       0
 mmu_recycled                 0       0
 mmu_shadow_zapped          269       0
 mmu_unsync                   0       0
 nmi_injections               0       0
 nmi_window                   0       0
 pf_fixed                 20630       0
 pf_guest                     0       0
 remote_tlb_flush           176       0
 request_irq                  0       0

Here -- http://www.corpit.ru/mjt/kvm-trace-w7-idle --
are some results from /sys/kernel/debug/tracing/events/kvm/enable
(taken from /sys/kernel/debug/tracing/trace_pipe).
The file's >5Mb in size.

When I run the same guest without -usbdevice tablet,
it idles at about 3..4% host CPU time -- still a bit
high but acceptable.

Here's a typical kvm_stats output withOUT usb tablet:

 efer_reload                  0       0
 exits                  1204081     686
 fpu_reload                4235       2
 halt_exits               12116      67
 halt_wakeup              12047      67
 host_state_reload       797865     493
 hypercalls                   0       0
 insn_emulation         1074589     473
 insn_emulation_fail          0       0
 invlpg                       0       0
 io_exits                761834     408
 irq_exits                22240      12
 irq_injections           33099       0
 irq_window                   0       0
 largepages                 304       0
 mmio_exits               16718       0
 mmu_cache_miss             220       0
 mmu_flooded                  0       0
 mmu_pde_zapped               0       0
 mmu_pte_updated              0       0
 mmu_pte_write                0       0
 mmu_recycled                 0       0
 mmu_shadow_zapped          271       0
 mmu_unsync                   0       0
 nmi_injections               0       0
 nmi_window                   0       0
 pf_fixed                 18790       0
 pf_guest                     0       0
 remote_tlb_flush           109       0
 request_irq                  0       0

And here's the two tables combined (last column from
the above two tables, without and with usbmouse):

statistics         without  with
efer_reload              0     0
exits                  686   695
fpu_reload               2     0
halt_exits              67    67
halt_wakeup             67    67
host_state_reload      493   495
hypercalls               0     0
insn_emulation         473   488
insn_emulation_fail      0     0
invlpg                   0     0
io_exits               408   409
irq_exits               12     8
irq_injections           0     0
irq_window               0     0
largepages               0     0
mmio_exits               0     0
mmu_cache_miss           0     0
mmu_flooded              0     0
mmu_pde_zapped           0     0
mmu_pte_updated          0     0
mmu_pte_write            0     0
mmu_recycled             0     0
mmu_shadow_zapped        0     0
mmu_unsync               0     0
nmi_injections           0     0
nmi_window               0     0
pf_fixed                 0     0
pf_guest                 0     0
remote_tlb_flush         0     0
request_irq              0     0

So basically they're quite similar to each other,
give or take some random fluctuations.

kvm version is 0.12.5, kvm command line:

 kvm -nodefaults -m 1G \
  -drive file=w7.raw,cache=none,boot=on,if=virtio \
  -monitor stdio \
  -vga std \
  -device virtio-net-pci,netdev=net \
  -netdev type=tap,ifname=tap-kvm,id=net
  -no-hpet
  -mem-path /dev/hugepages/

(and also -usbdevice tablet).  I especially disabled almost
everything (incl. hpet) and used "light" virtio drivers in
order to try to reduce load as much as possible.

The host CPU is AthlonII X2 260 (Regor).

Anyone know how to make w7 less idle-hungry for usb?

Thanks!

/mjt