Date: Tue, 12 Jun 2018 14:05:44 -0400 From: Mark Johnston <markj@freebsd.org> To: Warner Losh <imp@bsdimp.com> Cc: "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org> Subject: Re: specifying alignment of loader files Message-ID: <20180612180544.GG56138@raichu> In-Reply-To: <CANCZdfpDy44BSbs7n3HAyMSOTsrOQAAXrNfxsUrtAEOq5ACq4g@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>
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On Tue, Jun 12, 2018 at 11:55:27AM -0600, Warner Losh 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? It would (and that's actually what I did in my test branch). We already have to do this for ELF, and the only other frequently loaded file I can think of is /boot/entropy. Can you think of any scenarios where forcing page alignment for all files would waste a substantial amount of memory? If not, I might just go ahead with that approach for now and wait for a good reason before introducing customizable alignments.
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