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

[Pavel Dovgalyuk]

> From: Kevin Wolf [mailto:address@hidden]
> Am 19.09.2019 um 11:05 hat Pavel Dovgalyuk geschrieben:
> > > From: Kevin Wolf [mailto:address@hidden]
> > > > >
> > > > > 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;
> +    }

I guess this is opposite direction - bs->drv is bdrv_file.
And we should check its parent.

> +
>      bdrv_ref(bs);
>      blk->root = bdrv_root_attach_child(bs, "root", &child_root, blk->ctx,
>                                         blk->perm, blk->shared_perm, blk, 
> errp);

Pavel Dovgalyuk