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

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Date:   Thu, 28 Oct 2021 15:22:27 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Lai Jiangshan <jiangshanlai+lkml@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Ben Gardon <bgardon@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Liran Alon <liran.alon@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        John Haxby <john.haxby@...cle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v3 23/37] KVM: nVMX: Add helper to handle TLB flushes on
 nested VM-Enter/VM-Exit

-me :-)

On Thu, Oct 28, 2021, Lai Jiangshan wrote:
> On Sat, Mar 21, 2020 at 5:29 AM Sean Christopherson
> <sean.j.christopherson@...el.com> 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);
        }
 }