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 kvm@vger.
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];
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