[PATCH kvmtool v1 08/17] Use memfd for all guest ram allocations

[Fuad Tabba <tabba@google.com>]

Allocate all guest ram backed by memfd/ftruncate instead of
anonymous mmap. This will make it easier to use kvm with fd-based
kvm guest memory proposals [*]. It also would make it easier to
use ipc memory sharing should that be needed in the future.

Signed-off-by: Fuad Tabba <tabba@google.com>

[*] https://lore.kernel.org/all/20221025151344.3784230-1-chao.p.peng@linux.intel.com/
---
 include/kvm/kvm.h  |  1 +
 include/kvm/util.h |  3 +++
 kvm.c              |  4 ++++
 util/util.c        | 33 ++++++++++++++++++++-------------
 4 files changed, 28 insertions(+), 13 deletions(-)

diff --git a/include/kvm/kvm.h b/include/kvm/kvm.h
index 3872dc6..d0d519b 100644
--- a/include/kvm/kvm.h
+++ b/include/kvm/kvm.h
@@ -87,6 +87,7 @@ struct kvm {
 	struct kvm_config	cfg;
 	int			sys_fd;		/* For system ioctls(), i.e. /dev/kvm */
 	int			vm_fd;		/* For VM ioctls() */
+	int			ram_fd;		/* For guest memory. */
 	timer_t			timerid;	/* Posix timer for interrupts */
 
 	int			nrcpus;		/* Number of cpus to run */
diff --git a/include/kvm/util.h b/include/kvm/util.h
index 61a205b..369603b 100644
--- a/include/kvm/util.h
+++ b/include/kvm/util.h
@@ -140,6 +140,9 @@ static inline int pow2_size(unsigned long x)
 }
 
 struct kvm;
+int memfd_alloc(u64 size, bool hugetlb, u64 blk_size);
+void *mmap_anon_or_hugetlbfs_align(struct kvm *kvm, const char *htlbfs_path,
+				   u64 size, u64 align);
 void *mmap_anon_or_hugetlbfs(struct kvm *kvm, const char *htlbfs_path, u64 size);
 
 #endif /* KVM__UTIL_H */
diff --git a/kvm.c b/kvm.c
index 78bc0d8..ed29d68 100644
--- a/kvm.c
+++ b/kvm.c
@@ -160,6 +160,7 @@ struct kvm *kvm__new(void)
 	mutex_init(&kvm->mem_banks_lock);
 	kvm->sys_fd = -1;
 	kvm->vm_fd = -1;
+	kvm->ram_fd = -1;
 
 #ifdef KVM_BRLOCK_DEBUG
 	kvm->brlock_sem = (pthread_rwlock_t) PTHREAD_RWLOCK_INITIALIZER;
@@ -174,6 +175,9 @@ int kvm__exit(struct kvm *kvm)
 
 	kvm__arch_delete_ram(kvm);
 
+	if (kvm->ram_fd >= 0)
+		close(kvm->ram_fd);
+
 	list_for_each_entry_safe(bank, tmp, &kvm->mem_banks, list) {
 		list_del(&bank->list);
 		free(bank);
diff --git a/util/util.c b/util/util.c
index d3483d8..278bcc2 100644
--- a/util/util.c
+++ b/util/util.c
@@ -102,36 +102,38 @@ static u64 get_hugepage_blk_size(const char *htlbfs_path)
 	return sfs.f_bsize;
 }
 
-static void *mmap_hugetlbfs(struct kvm *kvm, const char *htlbfs_path, u64 size, u64 blk_size)
+int memfd_alloc(u64 size, bool hugetlb, u64 blk_size)
 {
 	const char *name = "kvmtool";
 	unsigned int flags = 0;
 	int fd;
-	void *addr;
-	int htsize = __builtin_ctzl(blk_size);
 
-	if ((1ULL << htsize) != blk_size)
-		die("Hugepage size must be a power of 2.\n");
+	if (hugetlb) {
+		int htsize = __builtin_ctzl(blk_size);
 
-	flags |= MFD_HUGETLB;
-	flags |= htsize << MFD_HUGE_SHIFT;
+		if ((1ULL << htsize) != blk_size)
+			die("Hugepage size must be a power of 2.\n");
+
+		flags |= MFD_HUGETLB;
+		flags |= htsize << MFD_HUGE_SHIFT;
+	}
 
 	fd = memfd_create(name, flags);
 	if (fd < 0)
-		die("Can't memfd_create for hugetlbfs map\n");
+		die("Can't memfd_create for memory map\n");
+
 	if (ftruncate(fd, size) < 0)
 		die("Can't ftruncate for mem mapping size %lld\n",
 			(unsigned long long)size);
-	addr = mmap(NULL, size, PROT_RW, MAP_PRIVATE, fd, 0);
-	close(fd);
 
-	return addr;
+	return fd;
 }
 
 /* This function wraps the decision between hugetlbfs map (if requested) or normal mmap */
 void *mmap_anon_or_hugetlbfs(struct kvm *kvm, const char *htlbfs_path, u64 size)
 {
 	u64 blk_size = 0;
+	int fd;
 
 	/*
 	 * We don't /need/ to map guest RAM from hugetlbfs, but we do so
@@ -146,9 +148,14 @@ void *mmap_anon_or_hugetlbfs(struct kvm *kvm, const char *htlbfs_path, u64 size)
 		}
 
 		kvm->ram_pagesize = blk_size;
-		return mmap_hugetlbfs(kvm, htlbfs_path, size, blk_size);
 	} else {
 		kvm->ram_pagesize = getpagesize();
-		return mmap(NULL, size, PROT_RW, MAP_ANON_NORESERVE, -1, 0);
 	}
+
+	fd = memfd_alloc(size, htlbfs_path, blk_size);
+	if (fd < 0)
+		return MAP_FAILED;
+
+	kvm->ram_fd = fd;
+	return mmap(NULL, size, PROT_RW, MAP_PRIVATE, kvm->ram_fd, 0);
 }
-- 
2.38.1.431.g37b22c650d-goog