From dfb9372702b2fb994392b8a6e8a39964c2656ae6 Mon Sep 17 00:00:00 2001
From: Chuan Zheng <[email protected]>
Date: Wed, 9 Feb 2022 08:49:41 +0800
Subject: [PATCH] migration: skip cache_drop for bios bootloader and nvram
 template

Qemu enabled page cache dropping for raw device on the destionation host
during shared storage migration.
However, fsync may take 300ms to multiple seconds to return in multiple-migration
scene, because all domains in a host share bios bootloader file, skip cache_drop
for bios bootloader and nvram template to avoid downtime increase.
---
 block.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/block.c b/block.c
index b7cb963929..3bfd4be6b4 100644
--- a/block.c
+++ b/block.c
@@ -68,6 +68,9 @@
 
 #define NOT_DONE 0x7fffffff /* used while emulated sync operation in progress */
 
+#define DEFAULT_BIOS_BOOT_LOADER_DIR "/usr/share/edk2"
+#define DEFAULT_NVRAM_TEMPLATE_DIR "/var/lib/libvirt/qemu/nvram"
+
 /* Protected by BQL */
 static QTAILQ_HEAD(, BlockDriverState) graph_bdrv_states =
     QTAILQ_HEAD_INITIALIZER(graph_bdrv_states);
@@ -7017,7 +7020,13 @@ int coroutine_fn bdrv_co_invalidate_cache(BlockDriverState *bs, Error **errp)
     assert(!(bs->open_flags & BDRV_O_INACTIVE));
     assert_bdrv_graph_readable();
 
-    if (bs->drv->bdrv_co_invalidate_cache) {
+    /*
+     * It's not necessary for bios bootloader and nvram template to drop cache
+     * when migration, skip this step for them to avoid dowtime increase.
+     */
+    if (bs->drv->bdrv_co_invalidate_cache &&
+        !strstr(bs->filename, DEFAULT_BIOS_BOOT_LOADER_DIR) &&
+        !strstr(bs->filename, DEFAULT_NVRAM_TEMPLATE_DIR)) {
         bs->drv->bdrv_co_invalidate_cache(bs, &local_err);
         if (local_err) {
             error_propagate(errp, local_err);
-- 
2.27.0