backends/hostmem: Report error when memory size is unaligned

If memory-backend-{file,ram} has a size that's not aligned to
underlying page size it is not only wasteful, but also may lead
to hard to debug behaviour. For instance, in case
memory-backend-file and hugepages, madvise() and mbind() fail.
Rightfully so, page is the smallest unit they can work with. And
even though an error is reported, the root cause it not very
clear:

  qemu-system-x86_64: Couldn't set property 'dump' on 'memory-backend-file': Invalid argument

After this commit:

  qemu-system-x86_64: backend 'memory-backend-file' memory size must be multiple of 2 MiB

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Message-ID: <b5b9f9c6bba07879fb43f3c6f496c69867ae3716.1717584048.git.mprivozn@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

backends/hostmem-epc.c

@@ -36,6 +36,7 @@ sgx_epc_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
         return false;
     }
 
+    backend->aligned = true;
     name = object_get_canonical_path(OBJECT(backend));
     ram_flags = (backend->share ? RAM_SHARED : 0) | RAM_PROTECTED;
     return memory_region_init_ram_from_fd(&backend->mr, OBJECT(backend), name,

backends/hostmem-file.c

@@ -80,6 +80,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
         g_assert_not_reached();
     }
 
+    backend->aligned = true;
     name = host_memory_backend_get_name(backend);
     ram_flags = backend->share ? RAM_SHARED : 0;
     ram_flags |= fb->readonly ? RAM_READONLY_FD : 0;

backends/hostmem-memfd.c

@@ -52,6 +52,7 @@ memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
         return false;
     }
 
+    backend->aligned = true;
     name = host_memory_backend_get_name(backend);
     ram_flags = backend->share ? RAM_SHARED : 0;
     ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;

backends/hostmem.c

@@ -20,6 +20,7 @@
 #include "qom/object_interfaces.h"
 #include "qemu/mmap-alloc.h"
 #include "qemu/madvise.h"
+#include "qemu/cutils.h"
 #include "hw/qdev-core.h"
 
 #ifdef CONFIG_NUMA
@@ -325,6 +326,7 @@ host_memory_backend_memory_complete(UserCreatable *uc, Error **errp)
     HostMemoryBackendClass *bc = MEMORY_BACKEND_GET_CLASS(uc);
     void *ptr;
     uint64_t sz;
+    size_t pagesize;
     bool async = !phase_check(PHASE_LATE_BACKENDS_CREATED);
 
     if (!bc->alloc) {
@@ -336,6 +338,14 @@ host_memory_backend_memory_complete(UserCreatable *uc, Error **errp)
 
     ptr = memory_region_get_ram_ptr(&backend->mr);
     sz = memory_region_size(&backend->mr);
+    pagesize = qemu_ram_pagesize(backend->mr.ram_block);
+
+    if (backend->aligned && !QEMU_IS_ALIGNED(sz, pagesize)) {
+        g_autofree char *pagesize_str = size_to_str(pagesize);
+        error_setg(errp, "backend '%s' memory size must be multiple of %s",
+                   object_get_typename(OBJECT(uc)), pagesize_str);
+        return;
+    }
 
     if (backend->merge) {
         qemu_madvise(ptr, sz, QEMU_MADV_MERGEABLE);

include/sysemu/hostmem.h

@@ -74,7 +74,7 @@ struct HostMemoryBackend {
     uint64_t size;
     bool merge, dump, use_canonical_path;
     bool prealloc, is_mapped, share, reserve;
-    bool guest_memfd;
+    bool guest_memfd, aligned;
     uint32_t prealloc_threads;
     ThreadContext *prealloc_context;
     DECLARE_BITMAP(host_nodes, MAX_NODES + 1);