Re: VMX and save/restore guest in virtual-8086 mode

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Apr 08, 2010 at 11:05:56AM +0300, Avi Kivity wrote:
> On 04/08/2010 10:54 AM, Jan Kiszka wrote:
> >
> >>>>Looks like KVM_SET_REGS should write rmode.save_iopl (and a new save_vm)?
> >>>>
> >>>Just like we manipulate the flags for guest debugging in the
> >>>set/get_rflags vendor handlers, the same should happen for IOPL and VM.
> >>>This is no business of enter_pmode/rmode.
> >>>
> >>This is vendor specific code, and it isn't manipulating guest values,
> >>only host values (->set_rflags() is called when the guest value changes,
> >>which isn't happening here).  Of course some refactoring will be helpful
> >>here.
> >Actually, the bug is that enter_pmode/rmode update save_iopl (and that
> >no one saves the VM bit). That should happen in vmx_set_rflags to also
> >keep track of changes _while_ we are in rmode.
> 
> Exactly - that's what I suggested above.

And new ioctl to save/restore save_iopl/save_vm.

> >enter_rmode/pmode should
> >just trigger a set_rflags to update things.
> 
> Not what I had in mind, but a valid implementation.
> 
> >And vmx_get_rflags must
> >properly inject the saved flags instead of masking them out.
> 
> Yes.  No one ever bothers to play with iopl in real mode, so we
> never noticed this.  We do this for cr0 for example.
> 
> >>It's a bugfix that can go into -stable and supported distribution kernels.
> >Well, would be happy to throw out tones of workaround based on this
> >approach. :)

Do you mean you'd be interested in writing the patch? Sure, go ahead,
let me know otherwise.

> And I'll be happy to apply such patches.  Just ensure that 2.6.32.y
> and above have the fixes so we don't introduce regressions (I think
> most workarounds are a lot older).
> 
> -- 
> error compiling committee.c: too many arguments to function