Re: [Xen-devel] [MULTIBOOT2 DOC PATCH 08/10] multiboot2: Add C structure alignment and padding consideration section

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Thu, Jun 09, 2016 at 11:07:12PM +0100, Andrew Cooper wrote:
> On 09/06/2016 21:30, Daniel Kiper wrote:
> > Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
> > ---
> >  doc/multiboot.texi |   17 +++++++++++++++++
> >  1 file changed, 17 insertions(+)
> >
> > diff --git a/doc/multiboot.texi b/doc/multiboot.texi
> > index c81b2ea..bf02a1b 100644
> > --- a/doc/multiboot.texi
> > +++ b/doc/multiboot.texi
> > @@ -1384,6 +1384,7 @@ document, but are included for prospective operating 
> > system and boot
> >  loader writers.
> >
> >  @menu
> > +* C structure alignment and padding consideration::
> >  * Notes on PC::
> >  * BIOS device mapping techniques::
> >  * Example OS code::
> > @@ -1391,6 +1392,22 @@ loader writers.
> >  @end menu
> >
> >
> > +@node C structure alignment and padding consideration
> > +@section C structure alignment and padding consideration
> > +
> > +Many C compilers try to optimize memory accesses aligning structure
>
> "by aligning"
>
> > +members properly. Usually they reach the goal by adding some padding.
>
> What does "properly" mean here?  The default padding will be specified
> by the default ABI the compiler conforms to.

Right. I do not want to go into the details in this section and duplicate
anything which is much better described somewhere else. So, that is why
I use "properly" here. However, if you think that it can be phrased
better then drop me a line.

> > +This is very useful thing in general. However, if you try to mix assembler
> > +with C or use C to implement structure low level access this behavior
> > +may lead, at least, to quite surprising results. Hence, compiler should
> > +be instructed to not optimize such accesses. Usually it is done by special
> > +attribute added to structure definition, e.g. GCC compatible sources use
> > +@samp{__attribute__ ((__packed__))} for this purpose. However, this is not
> > +required if it is known that its members are properly aligned and compiler
> > +does not do any optimization. Very good example of this is shown below in
> > +@file{multiboot2.h} file.
>
> I am not sure what you are trying to say.

Do you refer to whole paragraph or last sentence?

In general I would like to say that guys should pay attention to proper
usage of struct construct in C and be aware that bad things may happen when
they introduce new tags structs without __packed__ attribute. However, they
also should be aware that __packed__ is not always required. And tag structs
in multiboot2.h file does not contain __packed__ attribute because they are
build in proper way. I hope that helps.

Daniel

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