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

[Sean Christopherson <seanjc@google.com>]

On Thu, Dec 16, 2021, Lai Jiangshan wrote:
> 
> 
> 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@linux.alibaba.com>
> > > 
> > > 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@linux.alibaba.com>
> > > ---
> > >   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);

The better place for this is the path in vmx_set_cr0() that handles EPT + !URG.
I believe nested_vmx_restore_host_state(), which is used to restore host state if
hardware detects a VM-Fail that KVM misses on nested VM-Enter, is also missing a
call to mark CR3 dirty.

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 2f6f465e575f..b0c3eb80f5d5 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -4426,7 +4426,7 @@ static void nested_vmx_restore_host_state(struct kvm_vcpu *vcpu)

        nested_ept_uninit_mmu_context(vcpu);
        vcpu->arch.cr3 = vmcs_readl(GUEST_CR3);
-       kvm_register_mark_available(vcpu, VCPU_EXREG_CR3);
+       kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);

        /*
         * Use ept_save_pdptrs(vcpu) to load the MMU's cached PDPTRs
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 640f4719612c..1701cee96678 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -3066,8 +3066,10 @@ void vmx_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
                }

                /* Note, vmx_set_cr4() consumes the new vcpu->arch.cr0. */
-               if ((old_cr0_pg ^ cr0) & X86_CR0_PG)
+               if ((old_cr0_pg ^ cr0) & X86_CR0_PG) {
                        vmx_set_cr4(vcpu, kvm_read_cr4(vcpu));
+                       kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
+               }
        }

        /* depends on vcpu->arch.cr0 to be set to a new value */