Merge tag 'kvm-x86-mmu-6.9' of https://github.com/kvm-x86/linux into HEAD

KVM x86 MMU changes for 6.9:

 - Clean up code related to unprotecting shadow pages when retrying a guest
   instruction after failed #PF-induced emulation.

 - Zap TDP MMU roots at 4KiB granularity to minimize the delay in yielding if
   a reschedule is needed, e.g. if a high priority task needs to run.  Because
   KVM doesn't support yielding in the middle of processing a zapped non-leaf
   SPTE, zapping at 1GiB granularity can result in multi-millisecond lag when
   attempting to schedule in a high priority.

 - Rework TDP MMU root unload, free, and alloc to run with mmu_lock held for
   read, e.g. to avoid serializing vCPUs when userspace deletes a memslot.

 - Allocate write-tracking metadata on-demand to avoid the memory overhead when
   running kernels built with KVMGT support (external write-tracking enabled),
   but for workloads that don't use nested virtualization (shadow paging) or
   KVMGT.

1 files changed, 16 insertions, 23 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e6b1b85dca8a..064862d87b9e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8792,31 +8792,24 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 
 	kvm_release_pfn_clean(pfn);
 
-	/* The instructions are well-emulated on direct mmu. */
-	if (vcpu->arch.mmu->root_role.direct) {
-		unsigned int indirect_shadow_pages;
-
-		write_lock(&vcpu->kvm->mmu_lock);
-		indirect_shadow_pages = vcpu->kvm->arch.indirect_shadow_pages;
-		write_unlock(&vcpu->kvm->mmu_lock);
-
-		if (indirect_shadow_pages)
-			kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
-
-		return true;
-	}
-
 	/*
-	 * if emulation was due to access to shadowed page table
-	 * and it failed try to unshadow page and re-enter the
-	 * guest to let CPU execute the instruction.
+	 * If emulation may have been triggered by a write to a shadowed page
+	 * table, unprotect the gfn (zap any relevant SPTEs) and re-enter the
+	 * guest to let the CPU re-execute the instruction in the hope that the
+	 * CPU can cleanly execute the instruction that KVM failed to emulate.
 	 */
-	kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
-
-	/*
-	 * If the access faults on its page table, it can not
-	 * be fixed by unprotecting shadow page and it should
-	 * be reported to userspace.
+	if (vcpu->kvm->arch.indirect_shadow_pages)
+		kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
+
+	/*
+	 * If the failed instruction faulted on an access to page tables that
+	 * are used to translate any part of the instruction, KVM can't resolve
+	 * the issue by unprotecting the gfn, as zapping the shadow page will
+	 * result in the instruction taking a !PRESENT page fault and thus put
+	 * the vCPU into an infinite loop of page faults.  E.g. KVM will create
+	 * a SPTE and write-protect the gfn to resolve the !PRESENT fault, and
+	 * then zap the SPTE to unprotect the gfn, and then do it all over
+	 * again.  Report the error to userspace.
 	 */
 	return !(emulation_type & EMULTYPE_WRITE_PF_TO_SP);
 }