Re: [PATCH 06/16] vmx: nest: handling VMX instruction exits

[Keir Fraser <keir.fraser@eu.citrix.com>]

On 15/09/2010 09:15, "Christoph Egger" <Christoph.Egger@amd.com> wrote:

>> The main trick with x86_emulate extensions is determining the correct neat
>> small set of callback hooks to add, which is somewhat driven by deciding
>> what should be emulated within x86_emulate and what should be left without
>> for implementation in the caller's context.
> 
> There is a case where the host must emulate an instruction of the l2 guest
> when the l1 guest doesn't intercept it.
> 
> When the vcpu is in guest mode, the fields in struct hvm_vcpu and
> guest_cpu_user_regs() represent the l2 guest state in my patch series.
> 
> That way the instruction emulator works out-of-the box.

Well in this specific case, all VMX-related instructions executed by an L2
guest would properly cause vmexit to the L1 guest for emulation there. We
wouldn't want to emulate in Xen.

But yes I can see that emulation of L2 guest instructions is needed in some
other cases. Like instructions performing I/O in areas which L1 thinks it
has given L2 direct unmediated access to, but which Xen is actually
filtering or emulating.

 -- Keir

> You need to add instructions to the emulator that are missing there.