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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Allowing userspace to stop the guest with an emulation failure is a
possible denial of service, similar to L2 stopping L1 with an emulation
failure.

Paolo