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

[Maxim Levitsky <mlevitsk@redhat.com>]

On Wed, 2021-12-15 at 16:45 +0000, Sean Christopherson wrote:
> 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 */
> 

I tested this patch and my guests boot as well. I didn't test more stuff like migration or so,
will do tomorrow.

Best regards,
	Maxim Levitsky