Re: [PATCH] KVM: nVMX: Fix setting of CR0 and CR4 in guest mode

[Gleb Natapov <gleb@redhat.com>]

On Mon, Mar 04, 2013 at 03:25:47PM +0100, Jan Kiszka wrote:
> On 2013-03-04 15:15, Gleb Natapov wrote:
> > On Mon, Mar 04, 2013 at 03:09:51PM +0100, Jan Kiszka wrote:
> >> On 2013-03-04 14:22, Gleb Natapov wrote:
> >>> On Thu, Feb 28, 2013 at 10:44:47AM +0100, Jan Kiszka wrote:
> >>>> The logic for calculating the value with which we call kvm_set_cr0/4 was
> >>>> broken (will definitely be visible with nested unrestricted guest mode
> >>>> support). Also, we performed the check regarding CR0_ALWAYSON too early
> >>>> when in guest mode.
> >>>>
> >>>> What really needs to be done on both CR0 and CR4 is to mask out L1-owned
> >>>> bits and merge them in from GUEST_CR0/4. In contrast, arch.cr0/4 and
> >>>> arch.cr0/4_guest_owned_bits contain the mangled L0+L1 state and, thus,
> >>>> are not suited as input.
> >>>>
> >>>> For both CRs, we can then apply the check against VMXON_CRx_ALWAYSON and
> >>>> refuse the update if it fails. To be fully consistent, we implement this
> >>>> check now also for CR4.
> >>>>
> >>>> Finally, we have to set the shadow to the value L2 wanted to write
> >>>> originally.
> >>>>
> >>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> >>>> ---
> >>>>
> >>>> Found while making unrestricted guest mode working. Not sure what impact
> >>>> the bugs had on current feature level, if any.
> >>>>
> >>>> For interested folks, I've pushed my nEPT environment here:
> >>>>
> >>>>     git://git.kiszka.org/linux-kvm.git nept-hacking
> >>>>
> >>>>  arch/x86/kvm/vmx.c |   49 ++++++++++++++++++++++++++++++-------------------
> >>>>  1 files changed, 30 insertions(+), 19 deletions(-)
> >>>>
> >>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> >>>> index 7cc566b..d1dac08 100644
> >>>> --- a/arch/x86/kvm/vmx.c
> >>>> +++ b/arch/x86/kvm/vmx.c
> >>>> @@ -4605,37 +4605,48 @@ vmx_patch_hypercall(struct kvm_vcpu *vcpu, unsigned char *hypercall)
> >>>>  /* called to set cr0 as appropriate for a mov-to-cr0 exit. */
> >>>>  static int handle_set_cr0(struct kvm_vcpu *vcpu, unsigned long val)
> >>>>  {
> >>>> -	if (to_vmx(vcpu)->nested.vmxon &&
> >>>> -	    ((val & VMXON_CR0_ALWAYSON) != VMXON_CR0_ALWAYSON))
> >>>> -		return 1;
> >>>> -
> >>>>  	if (is_guest_mode(vcpu)) {
> >>>> -		/*
> >>>> -		 * We get here when L2 changed cr0 in a way that did not change
> >>>> -		 * any of L1's shadowed bits (see nested_vmx_exit_handled_cr),
> >>>> -		 * but did change L0 shadowed bits. This can currently happen
> >>>> -		 * with the TS bit: L0 may want to leave TS on (for lazy fpu
> >>>> -		 * loading) while pretending to allow the guest to change it.
> >>>> -		 */
> >>> Can't say I understand this patch yet, but it looks like the comment is
> >>> still valid. Why have you removed it?
> >>
> >> L0 allows L1 or L2 at most to own TS, the rest is host-owned. I think
> >> the comment was always misleading.
> >>
> > I do not see how it is misleading. For everything but TS we will not get
> > here (if L1 is kvm). For TS we will get here if L1 allows L2 to change
> > it, but L0 does not.
> 
> For everything *but guest-owned* we will get here, thus for most CR0
> accesses (bit-wise, not regarding frequency).
> 
I do not see how. If bit is trapped by L1 we will not get here. We will
do vmexit to L1 instead. nested_vmx_exit_handled_cr() check this condition.
I am not arguing about you code (didn't grok it yet), but the comment
still make sense to me.

--
			Gleb.