hostmem-file: add offset option

Add an option for hostmem-file to start the memory object at an offset
into the target file. This is useful if multiple memory objects reside
inside the same target file, such as a device node.

In particular, it's useful to map guest memory directly into /dev/mem
for experimentation.

To make this work consistently, also fix up all places in QEMU that
expect fd offsets to be 0.

Signed-off-by: Alexander Graf <graf@amazon.com>
Message-Id: <20230403221421.60877-1-graf@amazon.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

2 files changed, 14 insertions, 6 deletions

diff --git a/softmmu/memory.c b/softmmu/memory.c
index 9ee4131d75..7d9494ce70 100644
--- a/softmmu/memory.c
+++ b/softmmu/memory.c
@@ -1619,6 +1619,7 @@ void memory_region_init_ram_from_file(MemoryRegion *mr,
                                       uint64_t align,
                                       uint32_t ram_flags,
                                       const char *path,
+                                      ram_addr_t offset,
                                       bool readonly,
                                       Error **errp)
 {
@@ -1630,7 +1631,7 @@ void memory_region_init_ram_from_file(MemoryRegion *mr,
     mr->destructor = memory_region_destructor_ram;
     mr->align = align;
     mr->ram_block = qemu_ram_alloc_from_file(size, mr, ram_flags, path,
-                                             readonly, &err);
+                                             offset, readonly, &err);
     if (err) {
         mr->size = int128_zero();
         object_unparent(OBJECT(mr));
diff --git a/softmmu/physmem.c b/softmmu/physmem.c
index efaed36773..9d7e172260 100644
--- a/softmmu/physmem.c
+++ b/softmmu/physmem.c
@@ -1369,6 +1369,11 @@ static void *file_ram_alloc(RAMBlock *block,
         error_setg(errp, "alignment 0x%" PRIx64
                    " must be a power of two", block->mr->align);
         return NULL;
+    } else if (offset % block->page_size) {
+        error_setg(errp, "offset 0x%" PRIx64
+                   " must be multiples of page size 0x%zx",
+                   offset, block->page_size);
+        return NULL;
     }
     block->mr->align = MAX(block->page_size, block->mr->align);
 #if defined(__s390x__)
@@ -1400,7 +1405,7 @@ static void *file_ram_alloc(RAMBlock *block,
      * those labels. Therefore, extending the non-empty backend file
      * is disabled as well.
      */
-    if (truncate && ftruncate(fd, memory)) {
+    if (truncate && ftruncate(fd, offset + memory)) {
         perror("ftruncate");
     }
 
@@ -1416,6 +1421,7 @@ static void *file_ram_alloc(RAMBlock *block,
     }
 
     block->fd = fd;
+    block->fd_offset = offset;
     return area;
 }
 #endif
@@ -1889,7 +1895,7 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
 
     size = HOST_PAGE_ALIGN(size);
     file_size = get_file_size(fd);
-    if (file_size > 0 && file_size < size) {
+    if (file_size > offset && file_size < (offset + size)) {
         error_setg(errp, "backing store size 0x%" PRIx64
                    " does not match 'size' option 0x" RAM_ADDR_FMT,
                    file_size, size);
@@ -1929,7 +1935,7 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
 
 RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
                                    uint32_t ram_flags, const char *mem_path,
-                                   bool readonly, Error **errp)
+                                   off_t offset, bool readonly, Error **errp)
 {
     int fd;
     bool created;
@@ -1941,7 +1947,8 @@ RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
         return NULL;
     }
 
-    block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, 0, readonly, errp);
+    block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, offset, readonly,
+                                   errp);
     if (!block) {
         if (created) {
             unlink(mem_path);
@@ -2075,7 +2082,7 @@ void qemu_ram_remap(ram_addr_t addr, ram_addr_t length)
                 flags |= block->flags & RAM_NORESERVE ? MAP_NORESERVE : 0;
                 if (block->fd >= 0) {
                     area = mmap(vaddr, length, PROT_READ | PROT_WRITE,
-                                flags, block->fd, offset);
+                                flags, block->fd, offset + block->fd_offset);
                 } else {
                     flags |= MAP_ANONYMOUS;
                     area = mmap(vaddr, length, PROT_READ | PROT_WRITE,