Re: DOS VM problem with QEMU-KVM and newer kernels

[Gerhard Wiesinger <lists@wiesinger.com>]

On 15.04.2012 11:44, Avi Kivity wrote:
> On 04/12/2012 09:32 PM, Gerhard Wiesinger wrote:
>> Hello,
>>
>> I'm having problems with recents kernels and qemu-kvm with a DOS VM:
>> TD286
>> System: Bad selector: 0007
>> System: Bad selector: 0D87
>> System: Bad selector: 001F
>> System: Bad selector: 0007
>> GP at 0020 21D4 EC 0DC4
>> Error 269 loading D:\BP\BIN\TD286.EXE into extended memory
>>
>> Another 286 DOS Extender application also rises a general protection
>> fault:
>> GP at 0020 18A1 CODE 357C
>>
>> Doesn't depend on the used DOS memory manager and is always
>> reproduceable.
>>
>> Depends only on kernel version and not qemu-kvm and seabios (tried to
>> bisect it without success):
>> # NOK: Linux 3.3.1-3.fc16.x86_64 #1 SMP Wed Apr 4 18:08:51 UTC 2012
>> x86_64 x86_64 x86_64 GNU/Linux
>> # NOK: Linux 3.2.10-3.fc16.x86_64 #1 SMP Thu Mar 15 19:39:46 UTC 2012
>> x86_64 x86_64 x86_64 GNU/Linux
>> # OK: Linux 3.1.9-1.fc16.x86_64 #1 SMP Fri Jan 13 16:37:42 UTC 2012
>> x86_64 x86_64 x86_64 GNU/Linux
>> # OK: Linux 2.6.41.9-1.fc15.x86_64 #1 SMP Fri Jan 13 16:46:51 UTC 2012
>> x86_64 x86_64 x86_64 GNU/Linux
>>
>> CPU is an AMD one.
>>
>> Any ideas how to fix it again?
>> Any switches which might help?
>>
>
> The trigger is probably
>
>> commit f1c1da2bde712812a3e0f9a7a7ebe7a916a4b5f4
>> Author: Jan Kiszka<jan.kiszka@siemens.com>
>> Date:   Tue Oct 18 18:23:11 2011 +0200
>>
>>      KVM: SVM: Keep intercepting task switching with NPT enabled
>>
>>      AMD processors apparently have a bug in the hardware task switching
>>      support when NPT is enabled. If the task switch triggers a NPF, we can
>>      get wrong EXITINTINFO along with that fault. On resume, spurious
>>      exceptions may then be injected into the guest.
>>
>>      We were able to reproduce this bug when our guest triggered #SS
>> and the
>>      handler were supposed to run over a separate task with not yet touched
>>      stack pages.
>>
>>      Work around the issue by continuing to emulate task switches even in
>>      NPT mode.
>>
>>      Signed-off-by: Jan Kiszka<jan.kiszka@siemens.com>
>>      Signed-off-by: Marcelo Tosatti<mtosatti@redhat.com>
> Although it's not the patch's direct fault - it simply exposed an
> existing bug in kvm.
>
> Things to try:
> - revert the patch with a newer kernel
> - try 3.4-rc2 which has some task switch fixes from Kevin; if you want a
> Fedora kernel, use rawhide's [2]
> - post traces [1]
>
> Jan, Joerg, was an AMD erratum published for the bug?
>
> [1] http://www.linux-kvm.org/page/Tracing
> [2]
> http://mirrors.kernel.org/fedora/development/rawhide/x86_64/os/Packages/k/kernel-3.4.0-0.rc2.git2.1.fc18.x86_64.rpm
>

Hello Avi,

Status is as follows:
1.) Kernel 3.4.x DIDN'T fix the problem
2.) Reverting f1c1da2bde712812a3e0f9a7a7ebe7a916a4b5f4 FIXED the problem.

So the bug is still in 3.2., 3.3, 3.4rc present and a possible fix 
doesn't work. Should be fixed in 3.4 release.

How to proceed further?

I can try some patches if you want.

Thnx for all your help.

Ciao,
Gerhard