Re: [PATCHv3] VMX: Enhance invalid guest state emulation

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Sep 01, 2009 at 03:08:55PM +0200, Mohammed Gamal wrote:
> On Tue, Sep 1, 2009 at 2:18 PM, Marcelo Tosatti<mtosatti@redhat.com> wrote:
> > On Tue, Sep 01, 2009 at 02:14:17PM +0200, Mohammed Gamal wrote:
> >> On Tue, Sep 1, 2009 at 1:48 PM, Marcelo Tosatti<mtosatti@redhat.com> wrote:
> >> > On Tue, Sep 01, 2009 at 12:48:18PM +0200, Mohammed Gamal wrote:
> >> >> - Change returned handle_invalid_guest_state() to return relevant exit codes
> >> >> - Move triggering the emulation from vmx_vcpu_run() to vmx_handle_exit()
> >> >> - Return to userspace instead of repeatedly trying to emulate instructions that have already failed
> >> >>
> >> >> Signed-off-by: Mohammed Gamal <m.gamal005@gmail.com>
> >> >
> >> > Mohammed,
> >> >
> >> > The handle_invalid_guest_state loop is potentially problematic. It would
> >> > be more appropriate to use the __vcpu_run loop.
> >> >
> >> > Can't you set vmx->emulation_required depending on the result
> >> > of one call to emulate_instruction and get rid of the while
> >> > (!guest_state_valid(vcpu)) loop?
> >> >
> >>
> >> Invalid state emulation is VMX-specfic, while the __vcpu_run loop is
> >> independent of the virtualization extension (defined in x86.c), no?
> >> AMD SVM can comforably run hosts in big-real mode and thus it doesn't
> >> have the notion of a guest going to an invalid state because of mode
> >> switching, so I don't think it'd be a good idea to move emulation into
> >> a generic layer. Please correct me if I am wrong
> >
> > Right. But all i am asking is to emulate one instruction at a
> > time in handle_invalid_guest_state, instead of looping until
> > guest_state_valid(vcpu).
> >
> > So you get rid of schedule(), the check for signal_pending, etc.
> 
> But we'll still need to enter the guest when it's in a valid state, so
> we need to move that loop somewhere, 

Sure, just set vmx->emulation_required = guest_state_valid(vcpu). When
the state is good, the entry handler will vmentry.

> and now that we still have a loop
> we'll also still need to do the pending signals and scheduling checks,
> no?

Point is you can use the __vcpu_run loop.

In the latest patch you do:

+       /* Don't enter VMX if guest state is invalid, let the exit handler
+          start emulation until we arrive back to a valid state */
+       if (vmx->emulation_required && emulate_invalid_guest_state)
               	return;

And then emulate in the exit handler.

> I'd appreciate any suggestions you have to alleviate this.

I fail to see why you need an internal loop if you can use the external
(__vcpu_run) one.