Re: [Qemu-devel] [for-4.2 PATCH 3/6] replay: update docs for record/replay with block devices

[Kevin Wolf <kwolf@redhat.com>]

Am 19.09.2019 um 11:05 hat Pavel Dovgalyuk geschrieben:
> > From: Kevin Wolf [mailto:kwolf@redhat.com]
> > > >
> > > > However, global -snapshot is just a convenient shortcut for specifying
> > > > snapshot=on for all -drive arguments. So if -snapshot is incompatible
> > > > with replay, shouldn't manually marking all drives as snapshot=on be
> > > > incompatible as well?
> > > >
> > > > Maybe you're really interested in some specific drive not having
> > > > snapshot=on? But then it might be better to check that specific drive
> > > > instad of forbidding just the shortcut for setting it.
> > >
> > > -snapshot adds the flag for top-level drive, making driver operations
> > > dependent on temporary file structure.
> > >
> > > Moving this overlay beneath blkreplay driver makes drive operations
> > > deterministic for the top-level device.
> > 
> > So the real requirement is that blkreplay is the top-level node of any
> > guest device, right? And only because of this, you can't use -snapshot
> > (or snapshot=on on the blkreplay driver).
> > 
> > If we instead check e.g. in blk_insert_bs() or blk_attach_dev() that in
> > record/replay mode, the root node of the BlockBackend is blkreplay,
> > wouldn't we catch many more incorrect setups?
> 
> That sounds interesting.
> Will it help to check that every backend is connected to blkreplay?

Yes, it would return an error when you try to attach a non-blkreplay
node to a BlockBackend (and every guest device uses a BlockBackend).

Note that this restriction would currently make block jobs unavailable
on non-blkreplay nodes as they also use BlockBackends internally (though
this is going to change in the long run). I believe this restriction is
harmless and the typical replay use case doesn't involve any block jobs,
but if you do think it's a problem, blk_attach_dev() would be the place
that affects only devices.

> How then this check has to be done?

Only compile-tested, but maybe something like below?

Kevin

diff --git a/include/block/block_int.h b/include/block/block_int.h
index 0422acdf1c..9fa72bea51 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -955,6 +955,7 @@ static inline BlockDriverState *backing_bs(BlockDriverState *bs)
 extern BlockDriver bdrv_file;
 extern BlockDriver bdrv_raw;
 extern BlockDriver bdrv_qcow2;
+extern BlockDriver bdrv_blkreplay;
 
 int coroutine_fn bdrv_co_preadv(BdrvChild *child,
     int64_t offset, unsigned int bytes, QEMUIOVector *qiov,
diff --git a/block/blkreplay.c b/block/blkreplay.c
index 2b7931b940..16a4f1df6a 100644
--- a/block/blkreplay.c
+++ b/block/blkreplay.c
@@ -126,7 +126,7 @@ static int coroutine_fn blkreplay_co_flush(BlockDriverState *bs)
     return ret;
 }
 
-static BlockDriver bdrv_blkreplay = {
+BlockDriver bdrv_blkreplay = {
     .format_name            = "blkreplay",
     .instance_size          = 0,
 
diff --git a/block/block-backend.c b/block/block-backend.c
index 1c605d5444..c57d3d9fdf 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -17,6 +17,7 @@
 #include "block/throttle-groups.h"
 #include "hw/qdev-core.h"
 #include "sysemu/blockdev.h"
+#include "sysemu/replay.h"
 #include "sysemu/runstate.h"
 #include "qapi/error.h"
 #include "qapi/qapi-events-block.h"
@@ -808,6 +809,12 @@ void blk_remove_bs(BlockBackend *blk)
 int blk_insert_bs(BlockBackend *blk, BlockDriverState *bs, Error **errp)
 {
     ThrottleGroupMember *tgm = &blk->public.throttle_group_member;
+
+    if (replay_mode != REPLAY_MODE_NONE && bs->drv != &bdrv_blkreplay) {
+        error_setg(errp, "Root node must be blkreplay");
+        return -ENOTSUP;
+    }
+
     bdrv_ref(bs);
     blk->root = bdrv_root_attach_child(bs, "root", &child_root, blk->ctx,
                                        blk->perm, blk->shared_perm, blk, errp);