Re: [PATCH RFC] KVM: inject #UD if instruction emulation fails while vcpu is in cpl==3

[Avi Kivity <avi@redhat.com>]

On 05/06/2010 01:06 PM, Gleb Natapov wrote:
> On Thu, May 06, 2010 at 12:15:58PM +0300, Avi Kivity wrote:
>    
>> On 04/29/2010 02:58 PM, Gleb Natapov wrote:
>>      
>>> Do not kill VM If instruction emulation fails while vcpu is in
>>> userspace. Inject #UD instead in a hope that guest OS will kill offending
>>> process. Emulation failure is still traced by ftrace point to help
>>> analyze the problem.
>>>        
>> Still there's the risk here that a critical failure goes unnoticed.
>> ftrace isn't on at all times.
>>
>>      
> Kvm_stat will still show that there was emulation failure, so if strange
> application behaviour is reported kvm_stat output will have hints where
> to look. Next step in analyzing the problem will be enabling emulator
> tracing.
>    

We can expect that from a developer or a user subscribed to kvm@.  But 
what about some random user running virt-manager?

dmesg and kvm_stat will not go up the management stack.

>> We can probably inject a #UD unconditionally and exit to userspace.
>> Userspace would then report the problem to the user and reenter the
>> guest, which would then recover or not.
>>
>>      
> By "unconditionally" you mean even if guest is in kernel mode?

Yes.

> There is
> no point in trying to continue after that happens. Instead of getting
> paused VM at exact place where problem happened and easily analyzable we
> will get misbehaved VM with undefined state.
>    

True.  But the same problem exists with cpl>0 #UD.  It may be a critical 
driver in userspace (say, video driver).

Also need to think consider nested kernels (which are userspace for this 
purpose).

How about default to unconditional #UD and report, and pause if 
requested (in userspace)?  Usually emulation failures will be 100% 
reproducible, so the user can rerun their workload.

-- 
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