specifying alignment of loader files

[Kyle Evans]

On Tue, Jun 12, 2018 at 12:55 PM, Warner Losh <imp at bsdimp.com> wrote:
> On Tue, Jun 12, 2018 at 11:51 AM, Mark Johnston <markj at freebsd.org> wrote:
>
>> 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.
>>
>
> OK. That seems sane. I was thinking it was more general than this, but I'm
> cool with what you've outlined.
>
> OTOH, wouldn't just loading all files on page boundaries suffice?

If the answer to the above isn't "yes," can we consider making this a
flag to `load` parsed in command_load instead of its own
${module}_align variable? e.g.

cpu_ucode_name=/boot/firmware/...
cpu_ucode_flags="-a 16"

The initial recommendation isn't a bad idea,
but adding a new ${module}_flags requires changes in both interpreters
that I'm not sure are strictly necessary when this will be relatively
rarely used.

Thanks,

Kyle Evans