Re: [PATCH 11/15] KVM: VMX: Update vmcs.GUEST_CR3 only when the guest CR3 is dirty

[Lai Jiangshan]

On 2021/12/15 23:47, Maxim Levitsky wrote:
> On Mon, 2021-11-08 at 20:44 +0800, Lai Jiangshan wrote:
> > From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> >
> > When vcpu->arch.cr3 is changed, it is marked dirty, so vmcs.GUEST_CR3
> > can be updated only when kvm_register_is_dirty(vcpu, VCPU_EXREG_CR3).
> >
> > Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > ---
> >   arch/x86/kvm/vmx/vmx.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index d94e51e9c08f..38b65b97fb7b 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -3126,9 +3126,9 @@ static void vmx_load_mmu_pgd(struct kvm_vcpu *vcpu, hpa_t root_hpa,
> >   
> >   		if (!enable_unrestricted_guest && !is_paging(vcpu))
> >   			guest_cr3 = to_kvm_vmx(kvm)->ept_identity_map_addr;
> > -		else if (test_bit(VCPU_EXREG_CR3, (ulong *)&vcpu->arch.regs_avail))
> > +		else if (kvm_register_is_dirty(vcpu, VCPU_EXREG_CR3))
> >   			guest_cr3 = vcpu->arch.cr3;
> > -		else /* vmcs01.GUEST_CR3 is already up-to-date. */
> > +		else /* vmcs.GUEST_CR3 is already up-to-date. */
> >   			update_guest_cr3 = false;
> >   		vmx_ept_load_pdptrs(vcpu);
> >   	} else {
>
>
> I just bisected this patch to break booting a VM with ept=1 but unrestricted_guest=0
> (I needed to re-test unrestricted_guest=0 bug related to SMM, but didn't want
> to boot without EPT. With ept=0,the VM boots with this patch applied).


Thanks for reporting.

Sorry, I never tested it with unrestricted_guest=0. I can't reproduce it now shortly
with unrestricted_guest=0.  Maybe it can be reproduced easily if I try more guests or
I write a piece of guest code to deliberate hit it if the following analyses is correct.

All the paths changing %cr3 are followed with kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3)
and GUEST_CR3 will be expected to be updated.

What I missed is the case of "if (!enable_unrestricted_guest && !is_paging(vcpu))"
in vmx_load_mmu_pgd() which doesn't load GUEST_CR3 but clears dirty of VCPU_EXREG_CR3
(when after next run).

So when CR0 !PG -> PG, VCPU_EXREG_CR3 dirty bit should be set.

Maybe adding the following patch on top of the original patch can work.

Thanks
Lai

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 85127b3e3690..55b45005ebb9 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -858,6 +858,7 @@ void kvm_post_set_cr0(struct kvm_vcpu *vcpu, unsigned long old_cr0, unsigned lon
 	if ((cr0 ^ old_cr0) & X86_CR0_PG) {
 		kvm_clear_async_pf_completion_queue(vcpu);
 		kvm_async_pf_hash_reset(vcpu);
+		kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
 	}

 	if ((cr0 ^ old_cr0) & KVM_MMU_CR0_ROLE_BITS)