KVM: x86: Add platform hook for initializing private memory

TODO: add a CONFIG option rather than relying on a __weak reference for
arch/platform that don't implement this hook

All gmem pages are expected to be 'private' as defined by a particular
arch/platform. Platforms like SEV-SNP require additional operations to
move these pages into a private state, so implement a hook that can be
used to prepare this memory prior to mapping it into a guest.

In some cases, such as SEV-SNP, this memory needs to be written to prior
to converting it to a private page, so also add a __kvm_gmem_get_pfn()
helper that can be used to skip the hook.

Signed-off-by: Michael Roth <michael.roth@amd.com>

arch/x86/include/asm/kvm-x86-ops.h

@@ -133,6 +133,7 @@ KVM_X86_OP(msr_filter_changed)
 KVM_X86_OP(complete_emulated_msr)
 KVM_X86_OP(vcpu_deliver_sipi_vector)
 KVM_X86_OP_OPTIONAL_RET0(vcpu_get_apicv_inhibit_reasons);
+KVM_X86_OP_OPTIONAL_RET0(gmem_prepare)
 
 #undef KVM_X86_OP
 #undef KVM_X86_OP_OPTIONAL

arch/x86/include/asm/kvm_host.h

@@ -1732,6 +1732,9 @@ struct kvm_x86_ops {
 	 * Returns vCPU specific APICv inhibit reasons
 	 */
 	unsigned long (*vcpu_get_apicv_inhibit_reasons)(struct kvm_vcpu *vcpu);
+
+	int (*gmem_prepare)(struct kvm *kvm, struct kvm_memory_slot *slot,
+			    kvm_pfn_t pfn_start, gfn_t gfn_start, int order);
 };
 
 struct kvm_x86_nested_ops {

arch/x86/kvm/x86.c

@@ -13252,6 +13252,13 @@ bool kvm_arch_no_poll(struct kvm_vcpu *vcpu)
 }
 EXPORT_SYMBOL_GPL(kvm_arch_no_poll);
 
+#ifdef CONFIG_KVM_PRIVATE_MEM
+int kvm_arch_gmem_prepare(struct kvm *kvm, struct kvm_memory_slot *slot,
+			  kvm_pfn_t pfn_start, gfn_t gfn_start, int order)
+{
+	return static_call(kvm_x86_gmem_prepare)(kvm, slot, pfn_start, gfn_start, order);
+}
+#endif
 
 int kvm_spec_ctrl_test_value(u64 value)
 {

include/linux/kvm_host.h

@@ -2343,6 +2343,11 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
 #ifdef CONFIG_KVM_PRIVATE_MEM
 int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 			      gfn_t gfn, kvm_pfn_t *pfn, int *order);
+int __kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
+		       gfn_t gfn, kvm_pfn_t *pfn, int *order);
+int kvm_arch_gmem_prepare(struct kvm *kvm, struct kvm_memory_slot *slot,
+			  kvm_pfn_t pfn_start, gfn_t gfn_start, int order);
+
 #else
 static inline int kvm_gmem_get_pfn(struct kvm *kvm,
 				   struct kvm_memory_slot *slot, gfn_t gfn,
@@ -2351,6 +2356,14 @@ static inline int kvm_gmem_get_pfn(struct kvm *kvm,
 	KVM_BUG_ON(1, kvm);
 	return -EIO;
 }
+
+static inline int __kvm_gmem_get_pfn(struct kvm *kvm,
+				     struct kvm_memory_slot *slot, gfn_t gfn,
+				     kvm_pfn_t *pfn, int *order)
+{
+	KVM_BUG_ON(1, kvm);
+	return -EIO;
+}
 #endif /* CONFIG_KVM_PRIVATE_MEM */
 
 #endif

virt/kvm/guest_mem.c

@@ -469,9 +469,14 @@ void kvm_gmem_unbind(struct kvm_memory_slot *slot)
 	slot->gmem.file = NULL;
 }
 
+int __weak kvm_arch_gmem_prepare(struct kvm *kvm, struct kvm_memory_slot *slot,
+				 kvm_pfn_t pfn, gfn_t gfn, int order)
+{
+	return 0;
+}
 
-int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
-		     gfn_t gfn, kvm_pfn_t *pfn, int *order)
+int __kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
+		       gfn_t gfn, kvm_pfn_t *pfn, int *order)
 {
 	pgoff_t index = gfn - slot->base_gfn + slot->gmem.index;
 	struct file *file = slot->gmem.file;
@@ -495,6 +500,18 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 
 	return 0;
 }
+EXPORT_SYMBOL_GPL(__kvm_gmem_get_pfn);
+
+int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
+		     gfn_t gfn, kvm_pfn_t *pfn, int *order)
+{
+	int ret = __kvm_gmem_get_pfn(kvm, slot, gfn, pfn, order);
+
+	if (ret)
+		return ret;
+
+	return kvm_arch_gmem_prepare(kvm, slot, *pfn, gfn, *order);
+}
 EXPORT_SYMBOL_GPL(kvm_gmem_get_pfn);
 
 static int kvm_gmem_init_fs_context(struct fs_context *fc)