linux-kernel - Re: [PATCH v2 4/7] KVM: x86/mmu: Zap only obsolete roots if a root shadow page is zapped

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Message-ID: <217cc048-8ca7-2b7b-141f-f44f0d95eec5@redhat.com>
Date:   Wed, 2 Mar 2022 21:39:04 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.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,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH v2 4/7] KVM: x86/mmu: Zap only obsolete roots if a root
 shadow page is zapped

On 3/2/22 20:45, Sean Christopherson wrote:
> AMD NPT is hosed because KVM's awful ASID scheme doesn't assign an ASID per root
> and doesn't force a new ASID.  IMO, this is an SVM mess and not a TDP MMU bug.

I agree.

> In the short term, I think something like the following would suffice.  Long term,
> we really need to redo SVM ASID management so that ASIDs are tied to a KVM root.


diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index c5e3f219803e..7899ca4748c7 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3857,6 +3857,9 @@ static void svm_load_mmu_pgd(struct kvm_vcpu 
*vcpu, hpa_t root_hpa,
         unsigned long cr3;

         if (npt_enabled) {
+               if (is_tdp_mmu_root(root_hpa))
+                       svm->current_vmcb->asid_generation = 0;
+
                 svm->vmcb->control.nested_cr3 = __sme_set(root_hpa);
                 vmcb_mark_dirty(svm->vmcb, VMCB_NPT);

Why not just new_asid (even unconditionally, who cares)?

BTW yeah, the smoke test worked but the actual one failed horribly.

Paolo