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

Maxim Levitsky
2021-12-15T16:01:21Z

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).

Reverting this patch on top of kvm/queue also works.

Best regards,
Maxim levitsky