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 12:05:37PM +0100, Kevin Wolf wrote:
> >> +
> >> +	/*
> >> +	 * 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.
> 
> This is closely related to my argument with Gleb whether CPL changes
> when rflags is reloaded or if it only happens when cs is reloaded. I
> argued that it makes more sense to couple it with cs, so I guess I have
> to agree with you mostly.
> 
> The SDM says that any faults during loading the segment descriptors
> happen in the context of the new task, and the task switch is completed
> before an exception is generated. So it shouldn't make any difference to
> the guest what the exact point is at which we change CPL, it's an KVM
> internal decision.
> 
Actually no. What if fault happens during loading of CS descriptor? Spec
clearly says it should be performed in a new task context. Meaning in
vm86 mode. So cpl switch should happen already.

> So what about this order:
> 
> 1. Reload general purpose registers and eflags without updating mode
>    or writing back rflags to the vcpu struct.
> 
> 2. Load segment selectors without changing the DPL yet.
> 
> 3. Load segment descriptors, and disable the privilege checks in
>    load_segment_descriptor using a new flag. For SVM, this updates
>    the CPL when cs is reloaded.
> 
> 4. Call ctxt->ops.set_rflag so that VMX updates the CPL. Should be
>    cleaned up in a follow-up patch, so that VMX uses set_segment
>    to update the CPL, like SVM does.
> 
> Would this match your interpretation?
> 
> >> +	ctxt->ops->set_rflags(ctxt, ctxt->eflags);
> >> +
> >> +	/* General purpose registers */
> >>  	ctxt->regs[VCPU_REGS_RAX] = tss->eax;
> >>  	ctxt->regs[VCPU_REGS_RCX] = tss->ecx;
> >>  	ctxt->regs[VCPU_REGS_RDX] = tss->edx;
> >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> >> index dc3e945..502b5c3 100644
> >> --- a/arch/x86/kvm/x86.c
> >> +++ b/arch/x86/kvm/x86.c
> >> @@ -4040,6 +4040,11 @@ static int emulator_set_cr(struct x86_emulate_ctxt *ctxt, int cr, ulong val)
> >>  	return res;
> >>  }
> >>  
> >> +static void emulator_set_rflags(struct x86_emulate_ctxt *ctxt, ulong val)
> >> +{
> >> +	kvm_set_rflags(emul_to_vcpu(ctxt), val);
> >> +}
> >> +
> >>  static int emulator_get_cpl(struct x86_emulate_ctxt *ctxt)
> >>  {
> >>  	return kvm_x86_ops->get_cpl(emul_to_vcpu(ctxt));
> >> @@ -4199,6 +4204,7 @@ static struct x86_emulate_ops emulate_ops = {
> >>  	.set_idt	     = emulator_set_idt,
> >>  	.get_cr              = emulator_get_cr,
> >>  	.set_cr              = emulator_set_cr,
> >> +	.set_rflags          = emulator_set_rflags,
> >>  	.cpl                 = emulator_get_cpl,
> >>  	.get_dr              = emulator_get_dr,
> >>  	.set_dr              = emulator_set_dr,
> > 
> > It would be good to switch the entire emulator to use ->set_rflags()
> > instead of sometimes letting the caller do it.
> 
> If we change the CPL only with a cs reload, set_rflags can be dropped
> completely in the end.
> 
> Kevin

--
			Gleb.