Re: [PATCH] KVM/VMX: Check ept_pointer before flushing ept tlb

Vitaly Kuznetsov
2018-11-06T15:50:37Z

ltykernel@gmail.com writes:

> From: Lan Tianyu <Tianyu.Lan@microsoft.com>
>
> This patch is to initialize ept_pointer to INVALID_PAGE and check it
> before flushing ept tlb. If ept_pointer is invalidated, bypass the flush
> request.
>
> Signed-off-by: Lan Tianyu <Tianyu.Lan@microsoft.com>
> ---
> arch/x86/kvm/vmx.c | 16 +++++++++++++---
> 1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 4555077d69ce..edbc96cb990a 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -1580,14 +1580,22 @@ static int vmx_hv_remote_flush_tlb(struct kvm *kvm)
> /*
> * FLUSH_GUEST_PHYSICAL_ADDRESS_SPACE hypercall needs the address of the
> * base of EPT PML4 table, strip off EPT configuration information.
> + * If ept_pointer is invalid pointer, bypass the flush request.
> */
> if (to_kvm_vmx(kvm)->ept_pointers_match != EPT_POINTERS_MATCH) {
> - kvm_for_each_vcpu(i, vcpu, kvm)
> + kvm_for_each_vcpu(i, vcpu, kvm) {
> + if (!VALID_PAGE(to_vmx(vcpu)->ept_pointer))
> + return 0;
> +

To be honest I fail to understand the reason behind the patch: instead
of doing one unneeded flush request with ept_pointer==0 (after vCPU is
initialized) we now do the check every time. Could you please elaborate
on why this is needed?

> ret |= hyperv_flush_guest_mapping(
> - to_vmx(kvm_get_vcpu(kvm, i))->ept_pointer & PAGE_MASK);
> + to_vmx(vcpu)->ept_pointer & PAGE_MASK);

I would use a local variable for 'to_vmx(vcpu)->ept_pointer' or even
'to_vmx(vcpu)->ept_pointer & PAGE_MASK' and use it in VALID_PAGE() - as
lower bits are unrelated;

> + }
> } else {
> + if (!VALID_PAGE(to_vmx(kvm_get_vcpu(kvm, 0))->ept_pointer))
> + return 0;

Ditto.

> +
> ret = hyperv_flush_guest_mapping(
> - to_vmx(kvm_get_vcpu(kvm, 0))->ept_pointer & PAGE_MASK);
> + to_vmx(kvm_get_vcpu(kvm, 0))->ept_pointer & PAGE_MASK);

This doesn't belong to this patch.

> }
>
> spin_unlock(&to_kvm_vmx(kvm)->ept_pointer_lock);
> @@ -11568,6 +11576,8 @@ static struct kvm_vcpu *vmx_create_vcpu(struct kvm *kvm, unsigned int id)
> vmx->pi_desc.nv = POSTED_INTR_VECTOR;
> vmx->pi_desc.sn = 1;
>
> + vmx->ept_pointer = INVALID_PAGE;
> +
> return &vmx->vcpu;
>
> free_vmcs:

--
Vitaly