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

From: Paolo Bonzini
Date: Wed Mar 02 2022 - 15:39:20 EST


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