[Spice-devel] [PATCH spice-protocol] stats: Avoid holes in SpiceStat structure

[Christophe Fergeau]

On Thu, Jan 03, 2019 at 10:15:43AM -0500, Frediano Ziglio wrote:
> > 
> > On Thu, Jan 03, 2019 at 02:14:24PM +0000, Frediano Ziglio wrote:
> > > The SpiceStat structure can be 20 or 24 bytes depending on alignment.
> > > Being a memory mapped structure potentially used with lockless access
> > > is not good to have it unaligned.
> > > The current tool that reads this memory mapped file (reds_stats) is
> > > able to detect if the structure is either 20 or 24 bytes and act
> > > according so changing this structure won't affect the behaviour
> > > (unless you have an old tool but as they are usually packages together
> > > is quite improbable).
> > > This will also help on Windows as in that system is not possible for
> > > reds_stats to implement the same discovery trick implemented on Linux
> > > as on Windows is not possible to read the size of the file mapping
> > > (on Windows to implement shared memory you can use a file mapping
> > > not associated to a file).
> > > 
> > > Signed-off-by: Frediano Ziglio <fziglio at redhat.com>
> > > ---
> > >  spice/stats.h | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/spice/stats.h b/spice/stats.h
> > > index d4c45ab..2df90a3 100644
> > > --- a/spice/stats.h
> > > +++ b/spice/stats.h
> > > @@ -66,6 +66,8 @@ typedef struct SpiceStat {
> > >      uint32_t generation;
> > >      uint32_t num_of_nodes;
> > >      uint32_t root_index;
> > > +    /* to avoid holes in the structure on 64 bit */
> > > +    uint32_t padding;
> > 
> > I don't fully agree with the "on 64 bit" part of the comment, this is
> > more a matter of how the ABI is defined rather than something necessarily
> > tied to "64 bit".
> > 
> > Christophe
> > 
> 
> Well, to be more precise on platforms where a 64 bit integer is aligned to
> 64 bit.
> 
> Would:
> 
>    /* to avoid holes in the structure if uint64_t is aligned to 64 bits */
> 
> be better?

"Avoid holes in the structure on some platforms" maybe ?

Christophe
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