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[Qemu-devel] [PATCH v3] virtio-blk: set correct config size for the host

From: Changpeng Liu
Subject: [Qemu-devel] [PATCH v3] virtio-blk: set correct config size for the host driver
Date: Tue, 12 Feb 2019 23:19:49 +0800

Commit caa1ee43 "vhost-user-blk: add discard/write zeroes features
support" added fields to struct virtio_blk_config. This changes
the size of the config space and breaks migration from QEMU 3.1
and older:

qemu-system-ppc64: get_pci_config_device: Bad config data: i=0x10 read: 41 
device: 1 cmask: ff wmask: 80 w1cmask:0
qemu-system-ppc64: Failed to load PCIDevice:config
qemu-system-ppc64: Failed to load virtio-blk:virtio
qemu-system-ppc64: error while loading state for instance 0x0 of device 
qemu-system-ppc64: load of migration failed: Invalid argument

Since virtio-blk doesn't support the "discard" and "write zeroes"
features, it shouldn't even expose the associated fields in the
config space actually. Just include all fields up to num_queues to
match QEMU 3.1 and older.

Signed-off-by: Changpeng Liu <address@hidden>
 hw/block/virtio-blk.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 9a87b3b..0ff5315 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -28,6 +28,10 @@
 #include "hw/virtio/virtio-bus.h"
 #include "hw/virtio/virtio-access.h"
+/* We don't support discard yet, hide associated config fields. */
+#define VIRTIO_BLK_CFG_SIZE offsetof(struct virtio_blk_config, \
+                                     max_discard_sectors)
 static void virtio_blk_init_request(VirtIOBlock *s, VirtQueue *vq,
                                     VirtIOBlockReq *req)
@@ -761,7 +765,9 @@ static void virtio_blk_update_config(VirtIODevice *vdev, 
uint8_t *config)
     blkcfg.alignment_offset = 0;
     blkcfg.wce = blk_enable_write_cache(s->blk);
     virtio_stw_p(vdev, &blkcfg.num_queues, s->conf.num_queues);
-    memcpy(config, &blkcfg, sizeof(struct virtio_blk_config));
+    memcpy(config, &blkcfg, VIRTIO_BLK_CFG_SIZE);
+    QEMU_BUILD_BUG_ON(VIRTIO_BLK_CFG_SIZE > sizeof(struct virtio_blk_config));
 static void virtio_blk_set_config(VirtIODevice *vdev, const uint8_t *config)
@@ -769,7 +775,8 @@ static void virtio_blk_set_config(VirtIODevice *vdev, const 
uint8_t *config)
     VirtIOBlock *s = VIRTIO_BLK(vdev);
     struct virtio_blk_config blkcfg;
-    memcpy(&blkcfg, config, sizeof(blkcfg));
+    memcpy(&blkcfg, config, VIRTIO_BLK_CFG_SIZE);
+    QEMU_BUILD_BUG_ON(VIRTIO_BLK_CFG_SIZE > sizeof(blkcfg));
     blk_set_enable_write_cache(s->blk, blkcfg.wce != 0);
@@ -952,8 +959,7 @@ static void virtio_blk_device_realize(DeviceState *dev, 
Error **errp)
-    virtio_init(vdev, "virtio-blk", VIRTIO_ID_BLOCK,
-                sizeof(struct virtio_blk_config));
+    virtio_init(vdev, "virtio-blk", VIRTIO_ID_BLOCK, VIRTIO_BLK_CFG_SIZE);
     s->blk = conf->conf.blk;
     s->rq = NULL;

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