Re: [PATCH v3 23/37] KVM: nVMX: Add helper to handle TLB flushes on nested VM-Enter/VM-Exit

From: Sean Christopherson
Date: Thu Oct 28 2021 - 11:22:40 EST


-me :-)

On Thu, Oct 28, 2021, Lai Jiangshan wrote:
> On Sat, Mar 21, 2020 at 5:29 AM Sean Christopherson
> <sean.j.christopherson@xxxxxxxxx> wrote:
>
> > + if (!nested_cpu_has_vpid(vmcs12) || !nested_has_guest_tlb_tag(vcpu)) {
> > + kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
> > + } else if (is_vmenter &&
> > + vmcs12->virtual_processor_id != vmx->nested.last_vpid) {
> > + vmx->nested.last_vpid = vmcs12->virtual_processor_id;
> > + vpid_sync_context(nested_get_vpid02(vcpu));
> > + }
> > +}
>
> (I'm sorry to pick this old email to reply to, but the problem has
> nothing to do with this patch nor 5c614b3583e7 and it exists since
> nested vmx is introduced.)
>
> I think kvm_mmu_free_guest_mode_roots() should be called
> if (!enable_ept && vmcs12->virtual_processor_id != vmx->nested.last_vpid)
> just because prev_roots doesn't cache the vpid12.
> (prev_roots caches PCID, which is distinctive)
>
> The problem hardly exists if L1's hypervisor is also kvm, but if L1's
> hypervisor is different or is also kvm with some changes in the way how it
> manages VPID.

Indeed. A more straightforward error case would be if L1 and L2 share CR3, and
vmcs02.VPID is toggled (or used for the first time) on the L1 => L2 VM-Enter.

The fix should simply be:

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index eedcebf58004..574823370e7a 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -1202,17 +1202,15 @@ static void nested_vmx_transition_tlb_flush(struct kvm_vcpu *vcpu,
*
* If a TLB flush isn't required due to any of the above, and vpid12 is
* changing then the new "virtual" VPID (vpid12) will reuse the same
- * "real" VPID (vpid02), and so needs to be flushed. There's no direct
- * mapping between vpid02 and vpid12, vpid02 is per-vCPU and reused for
- * all nested vCPUs. Remember, a flush on VM-Enter does not invalidate
- * guest-physical mappings, so there is no need to sync the nEPT MMU.
+ * "real" VPID (vpid02), and so needs to be flushed. Like the !vpid02
+ * case above, this is a full TLB flush from the guest's perspective.
*/
if (!nested_has_guest_tlb_tag(vcpu)) {
kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
} else if (is_vmenter &&
vmcs12->virtual_processor_id != vmx->nested.last_vpid) {
vmx->nested.last_vpid = vmcs12->virtual_processor_id;
- vpid_sync_context(nested_get_vpid02(vcpu));
+ kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu);
}
}