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

[Peter Xu <peterx@redhat.com>]

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.

> 
> 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.

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..).

> 
> >
> > 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.

-- 
Peter Xu