Re: x86: Don't report guest userspace emulation error to userspace, why ?

[Bandan Das <bsd@redhat.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 09/12/2015 23:18, Bandan Das wrote:
>> Commit a2b9e6c1a35afcc09:
>> 
>>     KVM: x86: Don't report guest userspace emulation error to userspace
>>     
>>     Commit fc3a9157d314 ("KVM: X86: Don't report L2 emulation failures to
>>     user-space") disabled the reporting of L2 (nested guest) emulation failures to
>>     userspace due to race-condition between a vmexit and the instruction emulator.
>>     The same rational applies also to userspace applications that are permitted by
>>     the guest OS to access MMIO area or perform PIO.
>>     
>>     This patch extends the current behavior - of injecting a #UD instead of
>>     reporting it to userspace - also for guest userspace code.
>>     
>> I searched the archives but failed in finding anything. Can someone please
>> explain why this is needed ? Or, why not let userspace decide what to do based
>> on the cpl, whether to continue execution or kill the guest ? Is the assumption
>> here that this is what userspace always wants ?
>
> Not what userspace always wants, but what the guest kernel always wants.

Thanks Paolo, this one I agree.

> Allowing userspace to stop the guest with an emulation failure is a

This one I don't :) Userspace started the guest after all, there are other
ways for it to kill the guest if it wanted to.

> possible denial of service, similar to L2 stopping L1 with an emulation
> failure.
>
> Paolo
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