On 12/11/19 19:33, Liran Alon wrote:
> When KVM emulates a nested VMEntry (L1->L2 VMEntry), it switches mmu root
> page. If nEPT is used, this will happen from
> kvm_init_shadow_ept_mmu()->__kvm_mmu_new_cr3() and otherwise it will
> happpen from nested_vmx_load_cr3()->kvm_mmu_new_cr3(). Either case,
> __kvm_mmu_new_cr3() will use fast_cr3_switch() in attempt to switch to a
> previously cached root page.
>
> In case fast_cr3_switch() finds a matching cached root page, it will
> set it in mmu->root_hpa and request KVM_REQ_LOAD_CR3 such that on
> next entry to guest, KVM will set root HPA in appropriate hardware
> fields (e.g. vmcs->eptp). In addition, fast_cr3_switch() calls
> kvm_x86_ops->tlb_flush() in order to flush TLB as MMU root page
> was replaced.
>
> This works as mmu->root_hpa, which vmx_flush_tlb() use, was
> already replaced in cached_root_available(). However, this may
> result in unnecessary INVEPT execution because a KVM_REQ_TLB_FLUSH
> may have already been requested. For example, by prepare_vmcs02()
> in case L1 don't use VPID.
>
> Therefore, change fast_cr3_switch() to just request TLB flush on
> next entry to guest.
>
> Reviewed-by: Bhavesh Davda <bhavesh.davda@xxxxxxxxxx>
> Signed-off-by: Liran Alon <liran.alon@xxxxxxxxxx>
> ---
> arch/x86/kvm/mmu.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 24c23c66b226..150d982ec1d2 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -4295,7 +4295,7 @@ static bool fast_cr3_switch(struct kvm_vcpu *vcpu, gpa_t new_cr3,
> kvm_make_request(KVM_REQ_LOAD_CR3, vcpu);
> if (!skip_tlb_flush) {
> kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
> - kvm_x86_ops->tlb_flush(vcpu, true);
> + kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
> }
>
> /*
>
Queued, thanks.
(I should get kvm/queue properly tested and pushed by the end of this week).
Paolo
