Re: [Qemu-devel] qemu-kvm problem with DOS/4GW extender and EMM386.EXE

[Avi Kivity]

On 05/11/2010 11:56 PM, Andy Walls wrote:
> Running an MS-DOS 6.22 image with qemu-kvm on a RedHat Linux OS, I
> noticed the guest OS becomes hung and my dmesg gets spammed with
>
>         set_cr0: #GP, set PG flag with a clear PE flag
>
> That message appears to be the linux kernel's kvm emulator griping about
> Paging Enable bit being enabled while the Protection Enable bit is set
> for real mode.  (The Intel manual says this should be a protection
> fault).
>
> The program that causes this has the DOS/4GW DOS extender runtime
> compiled into it.
>
> I found that when I don't load the EMM386.EXE memory manager, the
> problem doesn't occur.
>
> Here's a kvmtrace segment of when things are not working:
>
>    

Please post kvm issues to address@hidden

> 0 (+           0)  CR_READ       vcpu = 0x00000000  pid = 0x00001997 [ CR# = 0, 
> value = 0x00000000 80000011 ]
> 28471049900815 (+        4000)  VMENTRY       vcpu = 0x00000000  pid = 
> 0x00001997
> 28471049903815 (+        3000)  VMEXIT        vcpu = 0x00000000  pid = 
> 0x00001997 [ exitcode = 0x00000010, rip = 0x00000000 00002a73 ]
> 0 (+           0)  LMSW          vcpu = 0x00000000  pid = 0x00001997 [ value = 
> 0x80000010 ]
> 28471049933815 (+       30000)  VMENTRY       vcpu = 0x00000000  pid = 
> 0x00001997
> 28471049936815 (+        3000)  VMEXIT        vcpu = 0x00000000  pid = 
> 0x00001997 [ exitcode = 0x0000007b, rip = 0x00000000 00001fd6 ]
>
>
>
>
> To me it appears EMM386.EXE enables paging, and the DOS/4GW DOS extender
> tries to manipulate the PE bit in CR0 with LMSW but doesn't succeed.
>
> These programs appear to work fine in VMWare and on real hardware.
>
>
> Any ideas on how to make EMM386.EXE and the DOS/$GW extender work in
> qemu-kvm?
>    

Looks like a bug in the implementation of LMSW.  The manual says:

> If the PE flag of the source operand (bit 0) is set to 1, the 
> instruction causes the
> processor to switch to protected mode. While in protected mode, the 
> LMSW instruc-
> tion cannot be used to clear the PE flag and force a switch back to 
> real-address mode.

But kvm doesn't implement that.  Instead, it follows the operation section:

> Operation
> CR0[0:3] ← SRC[0:3];

--
Do not meddle in the internals of kernels, for they are subtle and quick to 
panic.