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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-04-15 11:44, Avi Kivity wrote:
> 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?

It wasn't an erratum but a documented feature limitation in the AMD
architecture that was simply ignored by the old code.

Jan

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