KVM: Add support for read-only usage of gfn_to_page()

Rework gfn_to_page() to support read-only accesses so that it can be used
by arm64 to get MTE tags out of guest memory.

Opportunistically rewrite the comment to be even more stern about using
gfn_to_page(), as there are very few scenarios where requiring a struct
page is actually the right thing to do (though there are such scenarios).
Add a FIXME to call out that KVM probably should be pinning pages, not
just getting pages.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20241010182427.1434605-77-seanjc@google.com>

Author: sean-jc

include/linux/kvm_host.h

@@ -1213,7 +1213,12 @@ void kvm_arch_flush_shadow_memslot(struct kvm *kvm,
 int kvm_prefetch_pages(struct kvm_memory_slot *slot, gfn_t gfn,
 		       struct page **pages, int nr_pages);
 
-struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn);
+struct page *__gfn_to_page(struct kvm *kvm, gfn_t gfn, bool write);
+static inline struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn)
+{
+	return __gfn_to_page(kvm, gfn, true);
+}
+
 unsigned long gfn_to_hva(struct kvm *kvm, gfn_t gfn);
 unsigned long gfn_to_hva_prot(struct kvm *kvm, gfn_t gfn, bool *writable);
 unsigned long gfn_to_hva_memslot(struct kvm_memory_slot *slot, gfn_t gfn);

virt/kvm/kvm_main.c

@@ -3132,25 +3132,26 @@ int kvm_prefetch_pages(struct kvm_memory_slot *slot, gfn_t gfn,
 EXPORT_SYMBOL_GPL(kvm_prefetch_pages);
 
 /*
- * Do not use this helper unless you are absolutely certain the gfn _must_ be
- * backed by 'struct page'.  A valid example is if the backing memslot is
- * controlled by KVM.  Note, if the returned page is valid, it's refcount has
- * been elevated by gfn_to_pfn().
+ * Don't use this API unless you are absolutely, positively certain that KVM
+ * needs to get a struct page, e.g. to pin the page for firmware DMA.
+ *
+ * FIXME: Users of this API likely need to FOLL_PIN the page, not just elevate
+ *	  its refcount.
  */
-struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn)
+struct page *__gfn_to_page(struct kvm *kvm, gfn_t gfn, bool write)
 {
 	struct page *refcounted_page = NULL;
 	struct kvm_follow_pfn kfp = {
 		.slot = gfn_to_memslot(kvm, gfn),
 		.gfn = gfn,
-		.flags = FOLL_WRITE,
+		.flags = write ? FOLL_WRITE : 0,
 		.refcounted_page = &refcounted_page,
 	};
 
 	(void)kvm_follow_pfn(&kfp);
 	return refcounted_page;
 }
-EXPORT_SYMBOL_GPL(gfn_to_page);
+EXPORT_SYMBOL_GPL(__gfn_to_page);
 
 int __kvm_vcpu_map(struct kvm_vcpu *vcpu, gfn_t gfn, struct kvm_host_map *map,
 		   bool writable)