[Qemu-devel] [Bug 1658141] Re: QEMU's default msrs handling causes Windows 10 64 bit to crash

[Francois Gouget]

** Attachment added: "Executable to reproduce this bug"
   
https://bugs.launchpad.net/qemu/+bug/1658141/+attachment/4806830/+files/exception.exe.bz2

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Title:
  QEMU's default msrs handling causes Windows 10 64 bit to crash

Status in QEMU:
  New

Bug description:
  Wine uses QEMU to run its conformance test suite on Windows virtual
  machines. Wine's conformance tests check the behavior of various
  Windows APIs and verify that they behave as expected.

  One such test checks handling of exceptions down. When run on Windows 10 64 
bit in QEMU it triggers a "KMOD_EXCEPTION_NOT_HANDLED" BSOD in the VM. See:
  https://bugs.winehq.org/show_bug.cgi?id=40240

  
  To reproduce this bug:
  * Pick a Windows 10 64 bit VM on an Intel host.

  * Start the VM. I'm pretty sure any qemu command will do but here's what I 
used:
    qemu-system-x86_64 -machine pc-i440fx-2.1,accel=kvm -cpu core2duo,+nx -m 
2048 -hda /var/lib/libvirt/images/wtbw1064.qcow2

  * Grab the attached source code. The tar file is a bit big at 85KB
  because I had to include some Wine headers. However the source file
  proper, exception.c, is only 85 lines, including the LGPL header.

  * Compile the source code with MinGW by typing 'make'. This produces a
  32 bit exception.exe executable. I'll attach it for good measure.

  * Put exception.exe on the VM and run it.

  
  After investigation it turns out this happens:
   * Only for Windows 10 64 bit guests. Windows 10 32 bit and older Windows 
versions are unaffected.

   * Only on Intel hosts. At least both my Xeon E3-1226 v3 and i7-4790K
  hosts are impacted but not my Opteron 6128 one.

   * It does not seem to depend on the emulated CPU type: on the Intel hosts 
this happened with both 
  core2duo,nx and 'copy the host configuration' and did not depend on the 
number of emulated cpus/cores.

   * This happened with both QEMU 2.1 and 2.7, and both the 3.16.0 and
  4.8.11 Linux kernels, both on Debian 8.6 and Debian Testing.

  
  After searching for quite some time I discovered that the kvm kernel module 
was sneaking the following messages into /var/log/syslog precisely when the 
BSOD happens:

  Dec 16 13:43:48 vm3 kernel: [  191.624802] kvm [2064]: vcpu0, guest rIP: 
0xfffff803cb3c0bf3 kvm_set_msr_common: MSR_IA32_DEBUGCTLMSR 0x1, nop
  Dec 16 13:43:48 vm3 kernel: [  191.624835] kvm [2064]: vcpu0, guest rIP: 
0xfffff803cb3c0c5c unhandled rdmsr: 0x1c9

  A search on the Internet turned up a post suggesting to change kvm's
  ignore_msrs setting:

     echo 1 >/sys/module/kvm/parameters/ignore_msrs

  
https://www.reddit.com/r/VFIO/comments/42dj7n/some_games_crash_to_biosboot_on_launch/

  This does actually work and provides a workaround at least.

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