Re: [Xen-devel] [PATCH v3 08/10] nEPT: handle invept instruction from L1 VMM

["Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Thursday, December 20, 2012 5:55 PM
> To: Zhang, Xiantao
> Cc: Dong, Eddie; Nakajima, Jun; xen-devel@xxxxxxxxxxxxx; keir@xxxxxxx;
> tim@xxxxxxx
> Subject: Re: [PATCH v3 08/10] nEPT: handle invept instruction from L1 VMM
> 
> >>> On 20.12.12 at 16:43, Xiantao Zhang <xiantao.zhang@xxxxxxxxx> wrote:
> > --- a/xen/arch/x86/hvm/vmx/vmx.c
> > +++ b/xen/arch/x86/hvm/vmx/vmx.c
> > @@ -2573,10 +2573,14 @@ void vmx_vmexit_handler(struct
> cpu_user_regs *regs)
> >              update_guest_eip();
> >          break;
> >
> > +    case EXIT_REASON_INVEPT:
> > +        if ( nvmx_handle_invept(regs) == X86EMUL_OKAY )
> > +            update_guest_eip();
> > +        break;
> > +
> 
> I realize that you're just copying code written the same way elsewhere, but:
> What if (here and elsewhere) X86EMUL_OKAY is not being returned (e.g. in
> the non-nested case)? Without the nested VMX code, all of these would
> have ended up at the default case (crashing the guest). Iiuc the correct 
> action
> would be to inject an exception at least when X86EMUL_EXCEPTION is being
> returned here - whether that's done here or (perhaps better, as only it can
> know _what_ exception to inject) by the callee is another thing to decide.
> 
> Also, at the example of nvmx_handle_vmclear() I see that it produces
> exceptions in most of the cases, but I think all of the related code needs
> auditing that things are being handled consistently _and_ completely
> (constructs like
> 
>     if ( ... != X86EMUL_OKAY )
>         return X86EMUL_EXCEPTION;
> 
> are definitely not okay, as there are further X86EMUL_* values that can
> occur; if you know only the two must ever occur at a given place, ASSERT() so,
> making things clear to the reader without having to follow all code paths).
Hi, Jan
I think it is better that the callee should be responsible for handling the 
exception before returning X86EMUL_EXCEPTION to its caller. 
so for the newly-introduced two functions,  I will add the logic to handle its 
possible exceptions before its return.  
Xiantao




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