[Xen-devel] Re: [PATCH][RFC] Emulating real mode with x86_emulate

[Anthony Liguori <aliguori@xxxxxxxxxx>]

Kamble, Nitin A wrote:
> Hi Anthony,
>   With the new revision I am able to get the -emulate-16bit command 
> line switch working.
> (XEN) HVM2: Creating MP tables 
> ...                                             
> (XEN) HVM2: Loading Cirrus VGABIOS 
> ...                                         
> (XEN) HVM2: Loading ACPI 
> ...                                                   
> (XEN) HVM2: Loading VMXAssist ... 
> deadbeef                                     
> (XEN) HVM2: 
> foo                                                                
> (XEN) 
> hvmop_emulate_realmode                                                   
> (XEN) guest requests real mode 
> emulation                                       
> (XEN) foo 
> 221                                                                  
> (XEN) HVM2: Invoking ROMBIOS 
> ...                                               
> (XEN) hvm.c:446:d2 Triple fault on VCPU0 - invoking HVM system reset.
The Triple fault you're seeing here is terribly curious.  Also the 
"deadbeef" output.  Just to sanity check, I threw the following printk 
in vmcs.c
    while (!hypercall_preempt_check()) {
+        printk("eip = 0x%x\n", regs->eip);
        if (x86_emulate(&ctxt, &em_ops)) {

And I get the following output with a FC5 guest:

(XEN) 
hvmop_emulate_realmode                                                   
(XEN) guest requests real mode 
emulation                                       
(XEN) foo 
221                                                                  
(XEN) eip = 
0xd338d                                                            
(XEN) eip = 
0xd338e                                                            
(XEN) eip = 
0xffbf0000                                                         
(XEN) failed to emulate instruction at %eip = 
0xd338d                          
(XEN) domain_crash_sync called from 
vmcs.c:625                                 
(XEN) Domain 1 (vcpu#0) crashed on 
cpu#0:                                      
(XEN) ----[ Xen-3.0-unstable  x86_32  debug=n  Not tainted 
]----               
(XEN) CPU:    
0                                                                
(XEN) EIP:    
0010:[<000d338d>]                                                
(XEN) EFLAGS: 00000002   CONTEXT: 
hvm                                          
(XEN) eax: 00000076   ebx: 000d7324   ecx: 000d7324   edx: 
000000e9            
(XEN) esi: 000d4e54   edi: 000d3380   ebp: 000d72a8   esp: 
000d72a8            
(XEN) cr0: 00050032   cr4: 00000651   cr3: 00000000   cr2: 
00000000            
(XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: 0018   cs: 0010   

So, perhaps it's the guest you're using?  Clearly, we're running in 
x86_emulate and hitting a 16 bit instruction we can't handle.  N.B. the 
printk in the error path for x86_emulate is wrong.  I should be looking 
at regs->eip, not GUEST_RIP since that wouldn't have been updated again.
Regards,

Anthony Liguori

> I have not added any debug lines in the code yet, and I don't know how 
> far is the emulator going. I will check it tomorrow.
> Thanks & Regards,
> Nitin
> Open Source Technology Center, Intel Corporation.
> -------------------------------------------------------------------------
> The mind is like a parachute; it works much better when it's open.
>
>
> On Thu, 2007-03-29 at 17:59 -0700, Nitin A Kamble wrote:
> > On Thu, 2007-03-29 at 19:11 -0500, Anthony Liguori wrote:
> >
> > > Okay, you'll need to make sure that hvmloader and qemu are both being 
> > > rebuilt and reinstalled.
> > >       
> > I am sure both are built with the patch and installed correctly.
> >
> > > The really curious thing is that eip from the VMCS is set to 0.  eip 
> > > should be address of the EIP of whatever movl cr0 instruction that 
> > > occurs when returning from main() in hvmloader.
> > >       
> > I also find that x86_emulate() is never called while emulate_realmode 
> > is set in the hypervisor.
> > > I guess it's possible that if you are using a version of unstable that 
> > > predates Keir's recent hvmloader changes, you wouldn't have hvmloader 
> > > switching to real mode.  That could explain some of the weirdness you're 
> > > seeing.  What changeset are you on?
> > >       
> > So far I am using: r14560_20070326
> >
> > Now I am trying with: r14631_20070329
> >
> > Thanks & Regards,
> > Nitin
> > Open Source Technology Center, Intel Corporation.
> > -------------------------------------------------------------------------
> > The mind is like a parachute; it works much better when it's open.

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