Re: [PATCH 15/15] KVM: nVMX: Always write vmcs.GUEST_CR3 during nested VM-Exit

Paolo Bonzini
2021-11-18T15:52:35Z

On 11/8/21 13:44, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
>
> For VM-Enter, vmcs.GUEST_CR3 and vcpu->arch.cr3 are synced and it is
> better to mark VCPU_EXREG_CR3 available rather than dirty to reduce a
> redundant vmwrite(GUEST_CR3) in vmx_load_mmu_pgd().
>
> But nested_vmx_load_cr3() is also served for VM-Exit which doesn't
> set vmcs.GUEST_CR3.
>
> This patch moves writing to vmcs.GUEST_CR3 into nested_vmx_load_cr3()
> for both nested VM-Eneter/Exit and use kvm_register_mark_available().
>
> This patch doesn't cause any extra writing to vmcs.GUEST_CR3 and if
> userspace is modifying CR3 with KVM_SET_SREGS later, the dirty info
> for VCPU_EXREG_CR3 would be set for next writing to vmcs.GUEST_CR3
> and no update will be lost.
>
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> ---
> arch/x86/kvm/vmx/nested.c | 32 +++++++++++++++++++++-----------
> 1 file changed, 21 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index ee5a68c2ea3a..4ddd4b1b0503 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -1133,8 +1133,28 @@ static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3,
> if (!nested_ept)
> kvm_mmu_new_pgd(vcpu, cr3);
>
> + /*
> + * Immediately write vmcs.GUEST_CR3 when changing vcpu->arch.cr3.
> + *
> + * VCPU_EXREG_CR3 is marked available rather than dirty because
> + * vcpu->arch.cr3 and vmcs.GUEST_CR3 are synced when enable_ept and
> + * vmcs.GUEST_CR3 is irrelevant to vcpu->arch.cr3 when !enable_ept.
> + *
> + * For VM-Enter case, it will be propagated to vmcs12 on nested
> + * VM-Exit, which can occur without actually running L2 and thus
> + * without hitting vmx_load_mmu_pgd(), e.g. if L1 is entering L2 with
> + * vmcs12.GUEST_ACTIVITYSTATE=HLT, in which case KVM will intercept
> + * the transition to HLT instead of running L2.
> + *
> + * For VM-Exit case, it is likely that vmcs.GUEST_CR3 == cr3 here, but
> + * L1 may set HOST_CR3 to a value other than its CR3 before VM-Entry,
> + * so we just update it unconditionally.
> + */
> + if (enable_ept)
> + vmcs_writel(GUEST_CR3, cr3);
> +
> vcpu->arch.cr3 = cr3;
> - kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
> + kvm_register_mark_available(vcpu, VCPU_EXREG_CR3);
>
> /* Re-initialize the MMU, e.g. to pick up CR4 MMU role changes. */
> kvm_init_mmu(vcpu);
> @@ -2600,16 +2620,6 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
> from_vmentry, entry_failure_code))
> return -EINVAL;
>
> - /*
> - * Immediately write vmcs02.GUEST_CR3. It will be propagated to vmcs12
> - * on nested VM-Exit, which can occur without actually running L2 and
> - * thus without hitting vmx_load_mmu_pgd(), e.g. if L1 is entering L2 with
> - * vmcs12.GUEST_ACTIVITYSTATE=HLT, in which case KVM will intercept the
> - * transition to HLT instead of running L2.
> - */
> - if (enable_ept)
> - vmcs_writel(GUEST_CR3, vmcs12->guest_cr3);
> -
> /* Late preparation of GUEST_PDPTRs now that EFER and CRs are set. */
> if (load_guest_pdptrs_vmcs12 && nested_cpu_has_ept(vmcs12) &&
> is_pae_paging(vcpu)) {
>

I have to think more about this one and patch 17. I queued the rest.

Paolo