specifying alignment of loader files

[Mark Johnston]

On Tue, Jun 12, 2018 at 11:45:01AM -0600, Warner Losh wrote:
> On Tue, Jun 12, 2018 at 11:16 AM, Mark Johnston <markj at freebsd.org> wrote:
> 
> > On Tue, Jun 12, 2018 at 11:11:25AM -0600, Warner Losh wrote:
> > > On Tue, Jun 12, 2018 at 11:04 AM, Mark Johnston <markj at freebsd.org>
> > wrote:
> > >
> > > > Hi,
> > > >
> > > > I'm writing some code which processes a file loaded by the loader.  I
> > > > want the file's contents to be available at a certain alignment in
> > > > memory, and as far as I can see, the loader provides no alignment
> > > > guarantees today.  The access will happen early enough during boot that
> > > > making an aligned copy of the data will be awkward, so I'd like the
> > > > loader to provide the desired alignment.
> > > >
> > > > I'm considering adding a new "module_align" variable that would specify
> > > > the alignment for a given file type, and plumb that through to
> > > > command_load().  Does anyone have an alternate suggestion, or an
> > > > objection to my proposal?
> > > >
> > >
> > > I thought the loader already did that for ELF sections... Why not wrap
> > your
> > > file in such a segment?
> >
> > In this case it's a raw binary file (CPU microcode), and I want to be
> > able to load it without any modifications or wrappers.
> >
> 
> How do you identify the type then? I'm ok in theory with this (though a
> variable is more flexible than needed), but that's my main concern. What's
> the alignment required btw?

It'd be a property of the type, e.g.,

cpu_ucode_name=/boot/firmware/...
cpu_ucode_align=16

I do feel like having a separate variable is overkill, but I can't see
a less invasive solution that isn't hacky.

The required alignment for Intel is 16 bytes; I'm not yet sure whether
AMD has a required alignment.