Date: Tue, 12 Jun 2018 14:08:46 -0400 From: Mark Johnston <markj@freebsd.org> To: Kyle Evans <kevans@freebsd.org> Cc: Warner Losh <imp@bsdimp.com>, "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org> Subject: Re: specifying alignment of loader files Message-ID: <20180612180846.GH56138@raichu> In-Reply-To: <CACNAnaG4ZwOrqbhpHJaQQjg4HVMqiP=OqjEiF6GZOYQ1VXkkug@mail.gmail.com> References: <20180612170420.GD56138@raichu> <CANCZdfofkgg4NyGtVucn7O6r44q7JBqf1fHf0qqMbPT-sTui6A@mail.gmail.com> <20180612171649.GE56138@raichu> <CANCZdfqWyf2MoiS-KrZQZOV0LH4qg64vrcrdisLMBTDpeamMPw@mail.gmail.com> <20180612175130.GF56138@raichu> <CANCZdfpDy44BSbs7n3HAyMSOTsrOQAAXrNfxsUrtAEOq5ACq4g@mail.gmail.com> <CACNAnaG4ZwOrqbhpHJaQQjg4HVMqiP=OqjEiF6GZOYQ1VXkkug@mail.gmail.com>
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On Tue, Jun 12, 2018 at 01:00:06PM -0500, Kyle Evans wrote:
> On Tue, Jun 12, 2018 at 12:55 PM, Warner Losh <imp@bsdimp.com> wrote:
> > On Tue, Jun 12, 2018 at 11:51 AM, Mark Johnston <markj@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@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@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.
Oh, I didn't realize those flags get passed straight to command_load().
I think that's a better idea than what I suggested. I started this
thread when I realized that I would need to write some Forth code to
implement my idea. :)
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