Re: [PATCH 09/10] KVM: nVMX: Synchronize VMCS12 content with the shadow vmcs

[Gleb Natapov <gleb@redhat.com>]

On Wed, Apr 17, 2013 at 05:59:50PM +0300, Abel Gordon wrote:
> 
> 
> Gleb Natapov <gleb@redhat.com> wrote on 17/04/2013 05:34:49 PM:
> 
> > > @@ -5716,6 +5725,10 @@ static void nested_vmx_failValid(struct
> > >               X86_EFLAGS_SF | X86_EFLAGS_OF))
> > >           | X86_EFLAGS_ZF);
> > >     get_vmcs12(vcpu)->vm_instruction_error = vm_instruction_error;
> > > +   /*
> > > +    * We don't need to force a shadow sync because
> > > +    * VM_INSTRUCTION_ERROR is not shdowed
> > ---------------------------------------^ shadowed.
> > But lets just request sync. This is slow path anyway.
> 
> Why then ?
Because we do not care how fast the error case is handled, so no point
breaking the rules for it.

> Note this will require to call copy_shadow_to_vmcs12
> because nested_vmx_failValid can be called while L0 handles a L1
> exit (for vmx instruction emulation) and the shadow vmcs could
> have been modified by L1 before the exit.
> 
Right, not a big deal if this is the only case when it happens. When we
discussed accessors vs sync_shadow_vmcs flag approach I said that flag
will work only if no vmcs12 fields are changed not as part of vmexit or
vmwrite emulations. This one is such a field unfortunately. Hope it is
the only one. 

--
			Gleb.