Re: [Qemu-devel] [PATCH 02/19] block: Add BDS.backing_overridden

[Kevin Wolf]

Am 03.05.2016 um 12:49 hat Max Reitz geschrieben:
> On 02.05.2016 17:35, Kevin Wolf wrote:
> > Am 26.04.2016 um 23:32 hat Max Reitz geschrieben:
> >> If the backing file is overridden, this most probably does change the
> >> guest-visible data of a BDS. Therefore, we will need to consider this in
> >> bdrv_refresh_filename().
> >>
> >> Adding a new field to the BDS is not nice, but it is very simple and
> >> exactly keeps track of whether the backing file has been overridden.
> >>
> >> Signed-off-by: Max Reitz 
> > 
> > What about live snapshots or block jobs that manipulate bs->backing?
> > They don't use bdrv_open_backing_file(), but bdrv_append() or
> > bdrv_replace_in_backing_chain() in order to assign a backing file, which
> > may or may not be the same as in the image header.
> 
> I'll take a look into each case, thanks.
> 
> > Also, do we want to consider a backing file overridden when the user
> > specified it explicitly, but it matches the image header?
> 
> For simplicity's sake, I would. I think we've realized before that
> trying to identify whether to file names point to the same image is
> rather difficult.

I was just asking because in that case, we probably must consider things
like the backing file of a live snapshot overlay overridden as well (at
least when using an existing image).

Kevin


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Re: [Qemu-devel] [PATCH 02/19] block: Add BDS.backing_overridden

[Max Reitz]

On 02.05.2016 17:35, Kevin Wolf wrote:
> Am 26.04.2016 um 23:32 hat Max Reitz geschrieben:
>> If the backing file is overridden, this most probably does change the
>> guest-visible data of a BDS. Therefore, we will need to consider this in
>> bdrv_refresh_filename().
>>
>> Adding a new field to the BDS is not nice, but it is very simple and
>> exactly keeps track of whether the backing file has been overridden.
>>
>> Signed-off-by: Max Reitz 
> 
> What about live snapshots or block jobs that manipulate bs->backing?
> They don't use bdrv_open_backing_file(), but bdrv_append() or
> bdrv_replace_in_backing_chain() in order to assign a backing file, which
> may or may not be the same as in the image header.

I'll take a look into each case, thanks.

> Also, do we want to consider a backing file overridden when the user
> specified it explicitly, but it matches the image header?

For simplicity's sake, I would. I think we've realized before that
trying to identify whether to file names point to the same image is
rather difficult.

Max

> 
> Kevin
> 
>>  block.c   | 2 ++
>>  include/block/block_int.h | 1 +
>>  2 files changed, 3 insertions(+)
>>
>> diff --git a/block.c b/block.c
>> index e349e83..e178488 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -1299,6 +1299,7 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict 
>> *parent_options,
>>  
>>  reference = qdict_get_try_str(parent_options, bdref_key);
>>  if (reference || qdict_haskey(options, "file.filename")) {
>> +bs->backing_overridden = true;
>>  backing_filename[0] = '\0';
>>  } else if (bs->backing_file[0] == '\0' && qdict_size(options) == 0) {
>>  QDECREF(options);
>> @@ -1589,6 +1590,7 @@ static int bdrv_open_inherit(BlockDriverState **pbs, 
>> const char *filename,
>>  backing = qdict_get_try_str(options, "backing");
>>  if (backing && *backing == '\0') {
>>  flags |= BDRV_O_NO_BACKING;
>> +bs->backing_overridden = true;
>>  qdict_del(options, "backing");
>>  }
>>  
>> diff --git a/include/block/block_int.h b/include/block/block_int.h
>> index 10d8759..d73e9ce 100644
>> --- a/include/block/block_int.h
>> +++ b/include/block/block_int.h
>> @@ -409,6 +409,7 @@ struct BlockDriverState {
>>  char backing_file[PATH_MAX]; /* if non zero, the image is a diff of
>>  this file image */
>>  char backing_format[16]; /* if non-zero and backing_file exists */
>> +bool backing_overridden; /* backing file has been specified by the user 
>> */
>>  
>>  QDict *full_open_options;
>>  char exact_filename[PATH_MAX];
>> -- 
>> 2.8.0
>>




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Re: [Qemu-devel] [PATCH 02/19] block: Add BDS.backing_overridden

[Kevin Wolf]

Am 26.04.2016 um 23:32 hat Max Reitz geschrieben:
> If the backing file is overridden, this most probably does change the
> guest-visible data of a BDS. Therefore, we will need to consider this in
> bdrv_refresh_filename().
> 
> Adding a new field to the BDS is not nice, but it is very simple and
> exactly keeps track of whether the backing file has been overridden.
> 
> Signed-off-by: Max Reitz 

What about live snapshots or block jobs that manipulate bs->backing?
They don't use bdrv_open_backing_file(), but bdrv_append() or
bdrv_replace_in_backing_chain() in order to assign a backing file, which
may or may not be the same as in the image header.

Also, do we want to consider a backing file overridden when the user
specified it explicitly, but it matches the image header?

Kevin

>  block.c   | 2 ++
>  include/block/block_int.h | 1 +
>  2 files changed, 3 insertions(+)
> 
> diff --git a/block.c b/block.c
> index e349e83..e178488 100644
> --- a/block.c
> +++ b/block.c
> @@ -1299,6 +1299,7 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict 
> *parent_options,
>  
>  reference = qdict_get_try_str(parent_options, bdref_key);
>  if (reference || qdict_haskey(options, "file.filename")) {
> +bs->backing_overridden = true;
>  backing_filename[0] = '\0';
>  } else if (bs->backing_file[0] == '\0' && qdict_size(options) == 0) {
>  QDECREF(options);
> @@ -1589,6 +1590,7 @@ static int bdrv_open_inherit(BlockDriverState **pbs, 
> const char *filename,
>  backing = qdict_get_try_str(options, "backing");
>  if (backing && *backing == '\0') {
>  flags |= BDRV_O_NO_BACKING;
> +bs->backing_overridden = true;
>  qdict_del(options, "backing");
>  }
>  
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index 10d8759..d73e9ce 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -409,6 +409,7 @@ struct BlockDriverState {
>  char backing_file[PATH_MAX]; /* if non zero, the image is a diff of
>  this file image */
>  char backing_format[16]; /* if non-zero and backing_file exists */
> +bool backing_overridden; /* backing file has been specified by the user 
> */
>  
>  QDict *full_open_options;
>  char exact_filename[PATH_MAX];
> -- 
> 2.8.0
> 

Re: [Qemu-devel] [PATCH 02/19] block: Add BDS.backing_overridden

[Eric Blake]

On 04/26/2016 03:32 PM, Max Reitz wrote:
> If the backing file is overridden, this most probably does change the
> guest-visible data of a BDS. Therefore, we will need to consider this in
> bdrv_refresh_filename().
> 
> Adding a new field to the BDS is not nice, but it is very simple and
> exactly keeps track of whether the backing file has been overridden.
> 
> Signed-off-by: Max Reitz 
> ---
>  block.c   | 2 ++
>  include/block/block_int.h | 1 +
>  2 files changed, 3 insertions(+)

Reviewed-by: Eric Blake 

-- 
Eric Blake   eblake redhat com+1-919-301-3266
Libvirt virtualization library http://libvirt.org



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[Qemu-devel] [PATCH 02/19] block: Add BDS.backing_overridden

[Max Reitz]

If the backing file is overridden, this most probably does change the
guest-visible data of a BDS. Therefore, we will need to consider this in
bdrv_refresh_filename().

Adding a new field to the BDS is not nice, but it is very simple and
exactly keeps track of whether the backing file has been overridden.

Signed-off-by: Max Reitz 
---
 block.c   | 2 ++
 include/block/block_int.h | 1 +
 2 files changed, 3 insertions(+)

diff --git a/block.c b/block.c
index e349e83..e178488 100644
--- a/block.c
+++ b/block.c
@@ -1299,6 +1299,7 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict 
*parent_options,
 
 reference = qdict_get_try_str(parent_options, bdref_key);
 if (reference || qdict_haskey(options, "file.filename")) {
+bs->backing_overridden = true;
 backing_filename[0] = '\0';
 } else if (bs->backing_file[0] == '\0' && qdict_size(options) == 0) {
 QDECREF(options);
@@ -1589,6 +1590,7 @@ static int bdrv_open_inherit(BlockDriverState **pbs, 
const char *filename,
 backing = qdict_get_try_str(options, "backing");
 if (backing && *backing == '\0') {
 flags |= BDRV_O_NO_BACKING;
+bs->backing_overridden = true;
 qdict_del(options, "backing");
 }
 
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 10d8759..d73e9ce 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -409,6 +409,7 @@ struct BlockDriverState {
 char backing_file[PATH_MAX]; /* if non zero, the image is a diff of
 this file image */
 char backing_format[16]; /* if non-zero and backing_file exists */
+bool backing_overridden; /* backing file has been specified by the user */
 
 QDict *full_open_options;
 char exact_filename[PATH_MAX];
-- 
2.8.0