KVM: Use enum to track if cached PFN will be used in guest and/or host

Replace the guest_uses_pa and kernel_map booleans in the PFN cache code
with a unified enum/bitmask. Using explicit names makes it easier to
review and audit call sites.

Opportunistically add a WARN to prevent passing garbage; instantating a
cache without declaring its usage is either buggy or pointless.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220303154127.202856-2-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: sean-jc

arch/x86/kvm/xen.c

@@ -39,7 +39,7 @@ static int kvm_xen_shared_info_init(struct kvm *kvm, gfn_t gfn)
 	}
 
 	do {
-		ret = kvm_gfn_to_pfn_cache_init(kvm, gpc, NULL, false, true,
+		ret = kvm_gfn_to_pfn_cache_init(kvm, gpc, NULL, KVM_HOST_USES_PFN,
 						gpa, PAGE_SIZE, false);
 		if (ret)
 			goto out;

include/linux/kvm_host.h

@@ -1231,11 +1231,12 @@ void kvm_vcpu_mark_page_dirty(struct kvm_vcpu *vcpu, gfn_t gfn);
  * @gpc:	   struct gfn_to_pfn_cache object.
  * @vcpu:	   vCPU to be used for marking pages dirty and to be woken on
  *		   invalidation.
- * @guest_uses_pa: indicates that the resulting host physical PFN is used while
- *		   @vcpu is IN_GUEST_MODE; invalidations of the cache from MMU
- *		   notifiers (but not for KVM memslot changes!) will also force
- *		   @vcpu to exit the guest to refresh the cache.
- * @kernel_map:    requests a kernel virtual mapping (kmap / memremap).
+ * @usage:	   indicates if the resulting host physical PFN is used while
+ *		   the @vcpu is IN_GUEST_MODE (in which case invalidation of 
+ *		   the cache from MMU notifiers---but not for KVM memslot
+ *		   changes!---will also force @vcpu to exit the guest and
+ *		   refresh the cache); and/or if the PFN used directly
+ *		   by KVM (and thus needs a kernel virtual mapping).
  * @gpa:	   guest physical address to map.
  * @len:	   sanity check; the range being access must fit a single page.
  * @dirty:         mark the cache dirty immediately.
@@ -1250,9 +1251,8 @@ void kvm_vcpu_mark_page_dirty(struct kvm_vcpu *vcpu, gfn_t gfn);
  * accessing the target page.
  */
 int kvm_gfn_to_pfn_cache_init(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
-			      struct kvm_vcpu *vcpu, bool guest_uses_pa,
-			      bool kernel_map, gpa_t gpa, unsigned long len,
-			      bool dirty);
+			      struct kvm_vcpu *vcpu, enum pfn_cache_usage usage,
+			      gpa_t gpa, unsigned long len, bool dirty);
 
 /**
  * kvm_gfn_to_pfn_cache_check - check validity of a gfn_to_pfn_cache.

include/linux/kvm_types.h

@@ -18,6 +18,7 @@ struct kvm_memslots;
 
 enum kvm_mr_change;
 
+#include <linux/bits.h>
 #include <linux/types.h>
 #include <linux/spinlock_types.h>
 
@@ -46,6 +47,12 @@ typedef u64            hfn_t;
 
 typedef hfn_t kvm_pfn_t;
 
+enum pfn_cache_usage {
+	KVM_GUEST_USES_PFN = BIT(0),
+	KVM_HOST_USES_PFN  = BIT(1),
+	KVM_GUEST_AND_HOST_USE_PFN = KVM_GUEST_USES_PFN | KVM_HOST_USES_PFN,
+};
+
 struct gfn_to_hva_cache {
 	u64 generation;
 	gpa_t gpa;
@@ -64,11 +71,10 @@ struct gfn_to_pfn_cache {
 	rwlock_t lock;
 	void *khva;
 	kvm_pfn_t pfn;
+	enum pfn_cache_usage usage;
 	bool active;
 	bool valid;
 	bool dirty;
-	bool kernel_map;
-	bool guest_uses_pa;
 };
 
 #ifdef KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE

virt/kvm/pfncache.c

@@ -42,7 +42,7 @@ void gfn_to_pfn_cache_invalidate_start(struct kvm *kvm, unsigned long start,
 			 * If a guest vCPU could be using the physical address,
 			 * it needs to be forced out of guest mode.
 			 */
-			if (gpc->guest_uses_pa) {
+			if (gpc->usage & KVM_GUEST_USES_PFN) {
 				if (!evict_vcpus) {
 					evict_vcpus = true;
 					bitmap_zero(vcpu_bitmap, KVM_MAX_VCPUS);
@@ -224,7 +224,7 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
 			goto map_done;
 		}
 
-		if (gpc->kernel_map) {
+		if (gpc->usage & KVM_HOST_USES_PFN) {
 			if (new_pfn == old_pfn) {
 				new_khva = old_khva;
 				old_pfn = KVM_PFN_ERR_FAULT;
@@ -304,19 +304,19 @@ EXPORT_SYMBOL_GPL(kvm_gfn_to_pfn_cache_unmap);
 
 
 int kvm_gfn_to_pfn_cache_init(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
-			      struct kvm_vcpu *vcpu, bool guest_uses_pa,
-			      bool kernel_map, gpa_t gpa, unsigned long len,
-			      bool dirty)
+			      struct kvm_vcpu *vcpu, enum pfn_cache_usage usage,
+			      gpa_t gpa, unsigned long len, bool dirty)
 {
+	WARN_ON_ONCE(!usage || (usage & KVM_GUEST_AND_HOST_USE_PFN) != usage);
+
 	if (!gpc->active) {
 		rwlock_init(&gpc->lock);
 
 		gpc->khva = NULL;
 		gpc->pfn = KVM_PFN_ERR_FAULT;
 		gpc->uhva = KVM_HVA_ERR_BAD;
 		gpc->vcpu = vcpu;
-		gpc->kernel_map = kernel_map;
-		gpc->guest_uses_pa = guest_uses_pa;
+		gpc->usage = usage;
 		gpc->valid = false;
 		gpc->active = true;