PARAVIRT_GUEST bug? [was: Stable regression: usb-storage is stuck in 2.6.26.5]

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

Michael Tokarev wrote:

[usb-storage hangs on asus M2NPV-VM (nVidia GeForce 6150/nForce 430)
  motherboard with latest bios]

> Ok, it turned out to be pretty.. interesting.
> 
> It's not a regression within -stable kernel series.  The problem
> happens when BOTH are true:
> 
>   - using latest BIOS for this mobo (1401), AND
>   - enabling KVM_GUEST in kernel.
> 
> The only difference in my kernel config between .1 and .5 was
> to enable KVM_GUEST and KVM_CLOCK, and also I2C_HELPER_AUTO
> which was added by kernel update.
> 
> Going with previous BIOS (1201) OR disabling PARAVIRT_GUEST
> fixes the problem.  I'll try to figure out which config
> option is at problem here.

After seeing similar situation on another motherboard, I
become curious.

Here's the result:  M3A78-EM motherboard (also from Asus),
AMD780G/SB700 etc.  Updating to one of the latest bios
makes the system unbootable.  Kernel hangs on boot right
after displaying

  SLUB: Genslabs=12, HWalign=64, Order=0-1, MinObjects=4, CPUs=2, Nodes=1

Reverting to the older BIOS or disabling PARAVIRT_GUEST in kernel
fixes this (turning off KVM_GUEST isn't enough), and kernel happily
boots further, with next line being

  hpet clockevent registered
  ...

Reportedly, another Asus motherboard (M2N-SLI DELUXE) also hangs
with new BIOS and KVM_GUEST optimizations turned on, but not on
older BIOS.

So it's quite.. funny.  At least SOME latest BIOSes from Asustec,
together with PARAVIRT_GUEST, -- a problem.  Don't do either of
those, and it's all ok.

The question is if Asus introduced the same bug in quite.. some
of their recent BIOSes, or PARAVIRT_GUEST code is somehow buggy
and the bug is exposed by new BIOS code.

Any hints on this? ;)

Thanks!

/mjt