Date: Tue, 12 Jun 2018 11:13:16 -0700 From: Ben Widawsky <ben@bwidawsk.net> To: Brooks Davis <brooks@freebsd.org> Cc: Warner Losh <imp@bsdimp.com>, "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org>, Mark Johnston <markj@freebsd.org> Subject: Re: specifying alignment of loader files Message-ID: <20180612181316.e67bv44exkvhmtyv@mail.bwidawsk.net> In-Reply-To: <20180612181009.GA11238@spindle.one-eyed-alien.net> References: <20180612170420.GD56138@raichu> <CANCZdfofkgg4NyGtVucn7O6r44q7JBqf1fHf0qqMbPT-sTui6A@mail.gmail.com> <20180612171649.GE56138@raichu> <CANCZdfqWyf2MoiS-KrZQZOV0LH4qg64vrcrdisLMBTDpeamMPw@mail.gmail.com> <20180612175130.GF56138@raichu> <CANCZdfpDy44BSbs7n3HAyMSOTsrOQAAXrNfxsUrtAEOq5ACq4g@mail.gmail.com> <20180612181009.GA11238@spindle.one-eyed-alien.net>
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On 18-06-12 18:10:09, Brooks Davis wrote: > 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? > > Page boundaries might be too much on very small systems, but there's > a principled argument for 8-bytes just based on the aligment of > primative types so 16 (or even 64) isn't much of a stretch. 16-bytes > would future-proof for the CHERI variants that are likely to make > it to silicon. > > -- Brooks I think native cacheline size is a pretty safe yet more conservative than page size bet ie. me too for 64.
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