Re: [PATCH v2 3/3] KVM: x86 emulator: Allow PM/VM86 switch during task switch

[Gleb Natapov <gleb@redhat.com>]

On Mon, Jan 30, 2012 at 12:24:11PM +0200, Avi Kivity wrote:
> > +
> >  	ctxt->ops->set_segment(ctxt, selector, &desc, base3, seg);
> >  }
> >  
> > @@ -2273,6 +2281,24 @@ static int load_state_from_tss32(struct x86_emulate_ctxt *ctxt,
> >  		return emulate_gp(ctxt, 0);
> >  	ctxt->_eip = tss->eip;
> >  	ctxt->eflags = tss->eflags | 2;
> > +
> > +	/*
> > +	 * If we're switching between Protected Mode and VM86, we need to make
> > +	 * sure to update the mode before loading the segment descriptors so
> > +	 * that the selectors are interpreted correctly.
> > +	 *
> > +	 * Need to get it to the vcpu struct immediately because it influences
> > +	 * the CPL which is checked at least when loading the segment
> > +	 * descriptors and when pushing an error code to the new kernel stack.
> > +	 */
> > +	if (ctxt->eflags & X86_EFLAGS_VM)
> > +		ctxt->mode = X86EMUL_MODE_VM86;
> > +	else
> > +		ctxt->mode = X86EMUL_MODE_PROT32;
> > +
> 
> Shouldn't this be done after the set_segment_selector() block?  My
> interpretation of the SDM is that if a fault happens while loading
> descriptors the fault happens with old segment cache, that is, it needs
> to be interpreted according to the old mode.
> 
No, spec says:
Any errors associated with this loading and qualification occur in the
context of the new task.

--
			Gleb.