linux-kernel - [PATCH 11/11] KVM: x86: Add gmem hook for determining max NPT mapping level

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Date: Thu,  4 Apr 2024 14:50:33 -0400
From: Paolo Bonzini <pbonzini@...hat.com>
To: linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc: seanjc@...gle.com,
	michael.roth@....com,
	isaku.yamahata@...el.com
Subject: [PATCH 11/11] KVM: x86: Add gmem hook for determining max NPT mapping level

From: Michael Roth <michael.roth@....com>

In the case of SEV-SNP, whether or not a 2MB page can be mapped via a
2MB mapping in the guest's nested page table depends on whether or not
any subpages within the range have already been initialized as private
in the RMP table. The existing mixed-attribute tracking in KVM is
insufficient here, for instance:

  - gmem allocates 2MB page
  - guest issues PVALIDATE on 2MB page
  - guest later converts a subpage to shared
  - SNP host code issues PSMASH to split 2MB RMP mapping to 4K
  - KVM MMU splits NPT mapping to 4K

At this point there are no mixed attributes, and KVM would normally
allow for 2MB NPT mappings again, but this is actually not allowed
because the RMP table mappings are 4K and cannot be promoted on the
hypervisor side, so the NPT mappings must still be limited to 4K to
match this.

Add a hook to determine the max NPT mapping size in situations like
this.

Signed-off-by: Michael Roth <michael.roth@....com>
Message-Id: <20231230172351.574091-31-michael.roth@....com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/include/asm/kvm-x86-ops.h | 1 +
 arch/x86/include/asm/kvm_host.h    | 2 ++
 arch/x86/kvm/mmu/mmu.c             | 8 ++++++++
 3 files changed, 11 insertions(+)

diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
index c81990937ab4..2db87a6fd52a 100644
--- a/arch/x86/include/asm/kvm-x86-ops.h
+++ b/arch/x86/include/asm/kvm-x86-ops.h
@@ -140,6 +140,7 @@ KVM_X86_OP_OPTIONAL_RET0(vcpu_get_apicv_inhibit_reasons);
 KVM_X86_OP_OPTIONAL(get_untagged_addr)
 KVM_X86_OP_OPTIONAL(alloc_apic_backing_page)
 KVM_X86_OP_OPTIONAL_RET0(gmem_prepare)
+KVM_X86_OP_OPTIONAL_RET0(gmem_validate_fault)
 KVM_X86_OP_OPTIONAL(gmem_invalidate)
 
 #undef KVM_X86_OP
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 59c7b95034fc..67dc108dd366 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1811,6 +1811,8 @@ struct kvm_x86_ops {
 	void *(*alloc_apic_backing_page)(struct kvm_vcpu *vcpu);
 	int (*gmem_prepare)(struct kvm *kvm, kvm_pfn_t pfn, gfn_t gfn, int max_order);
 	void (*gmem_invalidate)(kvm_pfn_t start, kvm_pfn_t end);
+	int (*gmem_validate_fault)(struct kvm *kvm, kvm_pfn_t pfn, gfn_t gfn, bool is_private,
+				   u8 *max_level);
 };
 
 struct kvm_x86_nested_ops {
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 992e651540e8..13dd367b8af1 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4338,6 +4338,14 @@ static int kvm_faultin_pfn_private(struct kvm_vcpu *vcpu,
 			       fault->max_level);
 	fault->map_writable = !(fault->slot->flags & KVM_MEM_READONLY);
 
+	r = static_call(kvm_x86_gmem_validate_fault)(vcpu->kvm, fault->pfn,
+						     fault->gfn, fault->is_private,
+						     &fault->max_level);
+	if (r) {
+		kvm_release_pfn_clean(fault->pfn);
+		return r;
+	}
+
 	return RET_PF_CONTINUE;
 }
 
-- 
2.43.0