cr3 OOS optimisation breaks 32-bit GNU/kFreeBSD guest

[Aurelien Jarno <aurelien@aurel32.net>]

Hi,

Since kvm-81, I have noticed that GNU/kFreeBSD 32-bit guest are crashing
under high load (during a compilation for example) with the following 
error message:

| Fatal trap 12: page fault while in kernel mode
| fault virtual address   = 0x4
| fault code              = supervisor read, page not present
| instruction pointer     = 0x20:0xc0a4fc00
| stack pointer           = 0x28:0xe66d7a70
| frame pointer           = 0x28:0xe66d7a80
| code segment            = base 0x0, limit 0xfffff, type 0x1b
|                         = DPL 0, pres 1, def32 1, gran 1
| processor eflags        = interrupt enabled, resume, IOPL = 0
| current process         = 24037 (bash)
| trap number             = 12
| panic: page fault
| Uptime: 4m7s
| Cannot dump. No dump device defined.
| Automatic reboot in 15 seconds - press a key on the console to abort
 
I haven't tried yet with a plain FreeBSD guest, but I also expect it to
crash given the kernel (version 7.1) is almost the same. A closer 
investigation has shown that the following commit is causing the 
problem:

| commit 6364a3918cb5c28376849e7fca3e09bd66b859f3
| Author: Marcelo Tosatti <mtosatti@redhat.com>
| Date:   Mon Dec 1 22:32:04 2008 -0200
| 
|     KVM: MMU: skip global pgtables on sync due to cr3 switch
| 
|     Skip syncing global pages on cr3 switch (but not on cr4/cr0). This is
|     important for Linux 32-bit guests with PAE, where the kmap page is
|     marked as global.
| 
|     Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
|     Signed-off-by: Avi Kivity <avi@redhat.com>

As expected, loading the KVM module with oos_shadow=0 workaround the
problem. Please note that the guest is running in 32-bit mode, does not
use PAE, and uses global pages. My host has an Intel Q9450 CPU, and the
problem appears with both a 2.6.26 and a 2.6.28 64-bit kernel.

Does anybody see any problem in this patch? How can I further
debug the problem?

Aurelien

-- 
Aurelien Jarno	                        GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net