KVM: Allow page-sized MMU caches to be initialized with custom 64-bit values

Add support to MMU caches for initializing a page with a custom 64-bit
value, e.g. to pre-fill an entire page table with non-zero PTE values.
The functionality will be used by x86 to support Intel's TDX, which needs
to set bit 63 in all non-present PTEs in order to prevent !PRESENT page
faults from getting reflected into the guest (Intel's EPT Violation #VE
architecture made the less than brilliant decision of having the per-PTE
behavior be opt-out instead of opt-in).

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Message-Id: <5919f685f109a1b0ebc6bd8fc4536ee94bcc172d.1705965635.git.isaku.yamahata@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: sean-jc

include/linux/kvm_types.h

@@ -86,6 +86,7 @@ struct gfn_to_pfn_cache {
 struct kvm_mmu_memory_cache {
 	gfp_t gfp_zero;
 	gfp_t gfp_custom;
+	u64 init_value;
 	struct kmem_cache *kmem_cache;
 	int capacity;
 	int nobjs;

virt/kvm/kvm_main.c

@@ -401,12 +401,17 @@ static void kvm_flush_shadow_all(struct kvm *kvm)
 static inline void *mmu_memory_cache_alloc_obj(struct kvm_mmu_memory_cache *mc,
 					       gfp_t gfp_flags)
 {
+	void *page;
+
 	gfp_flags |= mc->gfp_zero;
 
 	if (mc->kmem_cache)
 		return kmem_cache_alloc(mc->kmem_cache, gfp_flags);
-	else
-		return (void *)__get_free_page(gfp_flags);
+
+	page = (void *)__get_free_page(gfp_flags);
+	if (page && mc->init_value)
+		memset64(page, mc->init_value, PAGE_SIZE / sizeof(u64));
+	return page;
 }
 
 int __kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int capacity, int min)
@@ -421,6 +426,13 @@ int __kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int capacity,
 		if (WARN_ON_ONCE(!capacity))
 			return -EIO;
 
+		/*
+		 * Custom init values can be used only for page allocations,
+		 * and obviously conflict with __GFP_ZERO.
+		 */
+		if (WARN_ON_ONCE(mc->init_value && (mc->kmem_cache || mc->gfp_zero)))
+			return -EIO;
+
 		mc->objects = kvmalloc_array(capacity, sizeof(void *), gfp);
 		if (!mc->objects)
 			return -ENOMEM;