Date: Tue, 12 Jun 2018 13:00:56 -0500 From: Kyle Evans <kevans@freebsd.org> Cc: Warner Losh <imp@bsdimp.com>, Mark Johnston <markj@freebsd.org>, "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org> Subject: Re: specifying alignment of loader files Message-ID: <CACNAnaH%2ByTNh2QcS7vMiaLLjGoHD5ZR%2Bh0mNwv2pbHfLvVTZOg@mail.gmail.com> 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 1:00 PM, Kyle Evans <kevans@freebsd.org> 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.
>
> Thanks,
>
> Kyle Evans
Sorry, that ${module}_flags should read ${module}_align.
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