Re: [PATCH 3/7] migration: Document the effect of vmstate_info_nullptr

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> On Wed, Jan 08, 2025 at 10:31:05AM -0300, Fabiano Rosas wrote:
>> Peter Xu <peterx@redhat.com> writes:
>> 
>> > On Tue, Jan 07, 2025 at 04:50:21PM -0300, Fabiano Rosas wrote:
>> >> The migration stream lacks magic numbers at some key points. It's easy
>> >> to mis-parse data. Unfortunately, the VMS_NULLPTR_MARKER continues
>> >> with the trend. A '0' byte is ambiguous and could be interpreted as a
>> >> valid 0x30.
>> >> 
>> >> It is maybe not worth trying to change this while keeping backward
>> >> compatibility, so add some words of documentation to clarify.
>> >> 
>> >> Signed-off-by: Fabiano Rosas <farosas@suse.de>
>> >> ---
>> >>  migration/vmstate-types.c    | 6 ++++++
>> >>  scripts/analyze-migration.py | 9 +++++++++
>> >>  2 files changed, 15 insertions(+)
>> >> 
>> >> diff --git a/migration/vmstate-types.c b/migration/vmstate-types.c
>> >> index e83bfccb9e..08ed059f87 100644
>> >> --- a/migration/vmstate-types.c
>> >> +++ b/migration/vmstate-types.c
>> >> @@ -339,6 +339,12 @@ static int put_nullptr(QEMUFile *f, void *pv, size_t size,
>> >>  
>> >>  const VMStateInfo vmstate_info_nullptr = {
>> >>      .name = "uint64",
>> >
>> > Ouch.. So I overlooked this line and this explains why it didn't go via
>> > VMSDFieldGeneric already.
>> 
>> Yes, actually I overlooked as well that it should match the size of the
>> data being handled in the get/put functions.
>> 
>> My comment below is about NULL -> 0x30 that I think should instead be
>> NULL -> 0x3030303030303030 so we have any chance of looking at this and
>> identifying it's a NULL pointer. When we write 0x30 it might become
>> confusing for people reading the scripts output that their stream has a
>> bunch of '0' in the place where pointers should be. If the MAGIC number
>> were more identifiable, I could change the script to output (null) or 0x0ULL.
>
> I suppose we can?  If we want, by renaming this from "uint64" to "nullptr",
> then add an entry for it in Python's vmsd_field_readers.

That would be a nice alternative because it maps NULL to something, just
like the actual stream does. NULL -> '0' in the stream, NULL -> nullptr
in the JSON. I'll give it a try, thanks.

>> 
>> We also don't really have the concept of a pointer, which I suspect
>> might be the real reason behind all this mess. So we'll see:
>> 
>> 0x30
>> 0x30
>> {
>>   .some
>>   .struct
>>   .here
>> }
>> 0x30
>> 
>> So all this patch was trying to do is document this situation somehow.
>
> Yes, more docs makes sense, though just to mention it's nothing better here
> to use a full size of pointer: firstly it's not possible I think as 32/64
> bits have different size of pointers...
>
> More importantly, we're not sending the pointer but a marker, in this case
> the size of the real pointer doesn't really matter, IMHO.  A marker would
> make sense in saving some bytes when / if the array is large and sparse.

Right, it's just that a larger data type allows for a more unique
marker, which can be detected more reliably by the consumers of the
stream. The smaller data type is too ambiguous.

>
> Said that, let's try above idea, maybe it's optimal as you said the script
> can show things like "nullptr" (or any better name, I think that's better
> than "null" at least to show it's not a real pointer, otherwise it's weird
> to see any pointer in a migration stream..).

Yes, the script is just presenting the data, we can use what's more
informative.

>
>> 
>> >
>> > Instead of below comment, do we still have chance to change this to
>> > something like "uint8"?  Then I suppose the script will be able to identify
>> > this properly.