[RFC PATCH v2 3/3] hw/block/pflash: use memory_region_init_rom_device_from_file()

[Philippe Mathieu-Daudé]

If the block drive is read-only we will model a "protected" flash
device. We can thus use memory_region_init_rom_device_from_file()
which mmap the backing file when creating the MemoryRegion.
If the same backing file is used by multiple QEMU instances, this
reduces the memory footprint (this is often the case with the
CODE flash image from OVMF and AAVMF).

Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 hw/block/pflash_cfi01.c | 39 +++++++++++++++++++++++++++++++--------
 1 file changed, 31 insertions(+), 8 deletions(-)

diff --git a/hw/block/pflash_cfi01.c b/hw/block/pflash_cfi01.c
index a5fa8d8b74a..ec290636298 100644
--- a/hw/block/pflash_cfi01.c
+++ b/hw/block/pflash_cfi01.c
@@ -702,6 +702,7 @@ static void pflash_cfi01_realize(DeviceState *dev, Error 
**errp)
     int ret;
     uint64_t blocks_per_device, sector_len_per_device, device_len;
     int num_devices;
+    bool romd_mr_shared_mapped;
 
     if (pfl->sector_len == 0) {
         error_setg(errp, "attribute \"sector-length\" not specified or zero.");
@@ -743,19 +744,41 @@ static void pflash_cfi01_realize(DeviceState *dev, Error 
**errp)
         pfl->ro = 0;
     }
 
-    memory_region_init_rom_device(
-        &pfl->mem, OBJECT(dev),
-        &pflash_cfi01_ops,
-        pfl,
-        pfl->name, total_len, errp);
-    if (*errp) {
-        return;
+    if (pfl->ro && pfl->blk) {
+        BlockDriverState *bs = blk_bs(pfl->blk);
+
+        /* If "raw" driver used, try to mmap the backing file as RAM_SHARED */
+        if (bs->drv == &bdrv_raw) { /* FIXME check offset=0 ? */
+            Error *local_err = NULL;
+
+            memory_region_init_rom_device_from_file(&pfl->mem, OBJECT(dev),
+                                                    &pflash_cfi01_ops, pfl,
+                                                    pfl->name, total_len,
+                                                    qemu_real_host_page_size,
+                                                    RAM_SHARED,
+                                                    bs->exact_filename,
+                                                    true, &local_err);
+            if (local_err) {
+                error_report_err(local_err);
+                /* fall back to memory_region_init_rom_device() */
+            } else {
+                romd_mr_shared_mapped = true;
+            }
+        }
+    }
+    if (!romd_mr_shared_mapped) {
+        memory_region_init_rom_device(&pfl->mem, OBJECT(dev),
+                                      &pflash_cfi01_ops, pfl,
+                                      pfl->name, total_len, errp);
+        if (*errp) {
+            return;
+        }
     }
 
     pfl->storage = memory_region_get_ram_ptr(&pfl->mem);
     sysbus_init_mmio(SYS_BUS_DEVICE(dev), &pfl->mem);
 
-    if (pfl->blk) {
+    if (pfl->blk && !romd_mr_shared_mapped) {
         if (!blk_check_size_and_read_all(pfl->blk, pfl->storage, total_len,
                                          errp)) {
             vmstate_unregister_ram(&pfl->mem, DEVICE(pfl));
-- 
2.26.2