Re: [PATCH] KVM: SVM: Keep intercepting task switching with NPT enabled

[Jan Kiszka <jan.kiszka@web.de>]

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On 2011-11-01 12:13, Roedel, Joerg wrote:
> On Tue, Oct 18, 2011 at 01:37:32PM -0400, Avi Kivity wrote:
>> On 10/18/2011 06:35 PM, Roedel, Joerg wrote:
>>> On Tue, Oct 18, 2011 at 12:23:11PM -0400, Jan Kiszka wrote:
>>>> 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>
>>>
>>> Acked-by: Joerg Roedel <joerg.roedel@amd.com>
>>>
>>
>> Will there be an erratum published for this bug?  Are all processors
>> affected?
>>
>> I'd like to limit this to just the buggy processors, if and when fixed
>> processors become available.
> 
> Actually, this isn't a bug. Please have a look at section 15.2.6 of the
> APM2:
> 
> 15.2.6 Restartable Instructions
> 
> SVM is designed to safely restart, with the exception of task switches,
> any intercepted instruction (either atomic or idempotent) after the
> intercept.
> 
> So SVM does not guarantee that an intercept during a task-switch can be
> restarted.

Not a bug but still a deficit as it makes hardware-assisted task
switching practically useless, no?

Jan


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