nixpkgs docs: Cross compilation docs

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+<chapter xmlns="http://docbook.org/ns/docbook"
+         xmlns:xlink="http://www.w3.org/1999/xlink"
+         xml:id="chap-cross">
+
+<title>Cross-compilation</title>
+
+<section xml:id="sec-cross-intro">
+  <title>Introduction</title>
+  <para>
+    "Cross-compilation" means compiling a program on one machine for another type of machine.
+    For example, a typical use of cross compilation is to compile programs for embedded devices.
+    These devices often don't have the computing power and memory to compile their own programs.
+    One might think that cross-compilation is a fairly niche concern, but there are advantages to being rigorous about distinguishing build-time vs run-time environments even when one is developing and deploying on the same machine.
+    Nixpkgs is increasingly adopting this opinion in that packages should be written with cross-compilation in mind, and nixpkgs should evaluate in a similar way (by minimizing cross-compilation-specific special cases) whether or not one is cross-compiling.
+  </para>
+
+  <para>
+    This chapter will be organized in three parts.
+    First, it will describe the basics of how to package software in a way that supports cross-compilation.
+    Second, it will describe how to use Nixpkgs when cross-compiling.
+    Third, it will describe the internal infrastructure supporting cross-compilation.
+  </para>
+</section>
+
+<!--============================================================-->
+
+<section xml:id="sec-cross-packaging">
+  <title>Packing in a cross-friendly manner</title>
+
+  <section>
+    <title>Platform parameters</title>
+    <para>
+      The three GNU Autoconf platforms, <wordasword>build</wordasword>, <wordasword>host</wordasword>, and <wordasword>cross</wordasword>, are historically the result of much confusion.
+      <link xlink:href="https://gcc.gnu.org/onlinedocs/gccint/Configure-Terms.html" /> clears this up somewhat but there is more to be said.
+      An important advice to get out the way is, unless you are packaging a compiler or other build tool, just worry about the build and host platforms.
+      Dealing with just two platforms usually better matches people's preconceptions, and in this case is completely correct.
+    </para>
+    <para>
+      In Nixpkgs, these three platforms are defined as attribute sets under the names <literal>buildPlatform</literal>, <literal>hostPlatform</literal>, and <literal>targetPlatform</literal>.
+      All are guaranteed to contain at least a <varname>platform</varname> field, which contains detailed information on the platform.
+      All three are always defined at the top level, so one can get at them just like a dependency in a function that is imported with <literal>callPackage</literal>:
+      <programlisting>{ stdenv, buildPlatform, hostPlatform, fooDep, barDep, .. }: ...</programlisting>
+    </para>
+    <warning><para>
+      These platforms should all have the same structure in all scenarios, but that is currently not the case.
+      When not cross-compiling, they will each contain a <literal>system</literal> field with a short 2-part, hyphen-separated summering string name for the platform.
+      But, when when cross compiling, <literal>hostPlatform</literal> and <literal>targetPlatform</literal> may instead contain <literal>config</literal> with a fuller 3- or 4-part string in the manner of LLVM.
+      We should have all 3 platforms always contain both, and maybe give <literal>config</literal> a better name while we are at it.
+    </para></warning>
+    <variablelist>
+      <varlistentry>
+        <term><varname>buildPlatform</varname></term>
+        <listitem><para>
+          The "build platform" is the platform on which a package is built.
+          Once someone has a built package, or pre-built binary package, the build platform should not matter and be safe to ignore.
+        </para></listitem>
+      </varlistentry>
+      <varlistentry>
+        <term><varname>hostPlatform</varname></term>
+        <listitem><para>
+          The "host platform" is the platform on which a package is run.
+          This is the simplest platform to understand, but also the one with the worst name.
+        </para></listitem>
+      </varlistentry>
+      <varlistentry>
+        <term><varname>targetPlatform</varname></term>
+        <listitem>
+          <para>
+            The "target platform" is black sheep.
+            The other two intrinsically apply to all compiled software—or any build process with a notion of "build-time" followed by "run-time".
+            The target platform only applies to programming tools, and even then only is a good for for some of them.
+            Briefly, GCC, Binutils, GHC, and certain other tools are written in such a way such that a single build can only compiler code for a single platform.
+            Thus, when building them, one must think ahead about what platforms they wish to use the tool to produce machine code for, and build binaries for each.
+          </para>
+          <para>
+            There is no fundamental need to think about the target ahead of time like this.
+            LLVM, for example, was designed from the beginning with cross-compilation in mind, and so a normal LLVM binary will support every architecture that LLVM supports.
+            If the tool supports modular or pluggable backends, one might imagine specifying a <emphasis>set</emphasis> of target platforms / backends one wishes to support, rather than a single one.
+          </para>
+          <para>
+            The biggest reason for mess, if there is one, is that many compilers have the bad habit a build process that builds the compiler and standard library/runtime together.
+            Then the specifying target platform is essential, because it determines the host platform of the standard library/runtime.
+            Nixpkgs tries to avoid this where possible too, but still, because the concept of a target platform is so ingrained now in Autoconf and other tools, it is best to support it as is.
+            Tools like LLVM that don't need up-front target platforms can safely ignore it like normal packages, and it will do no harm.
+          </para>
+          </listitem>
+      </varlistentry>
+    </variablelist>
+    <note><para>
+      If you dig around nixpkgs, you may notice there is also <varname>stdenv.cross</varname>.
+      This field defined as <varname>hostPlatform</varname> when the host and build platforms differ, but otherwise not defined at all.
+      This field is obsolete and will soon disappear—please do not use it.
+    </para></note>
+  </section>
+
+  <section>
+    <title>Specifying Dependencies</title>
+    <para>
+      As mentioned in the introduction to this chapter, one can think about a build time vs run time distinction whether cross-compiling or not.
+      In the case of cross-compilation, this corresponds with whether a derivation running on the native or foreign platform is produced.
+      An interesting thing to think about is how this corresponds with the three Autoconf platforms.
+      In the run-time case, the depending and depended-on package simply have matching build, host, and target platforms.
+      But in the build-time case, one can imagine "sliding" the platforms one over.
+      The depended-on package's host and target platforms (respectively) become the depending package's build and host platforms.
+      This is the most important guiding principle behind cross-compilation with Nixpkgs, and will be called the <wordasword>sliding window principle</wordasword>.
+      In this manner, given the 3 platforms for one package, we can determine the three platforms for all its transitive dependencies.
+    </para>
+    <note><para>
+      The depending package's target platform is unconstrained by the sliding window principle, which makes sense in that one can in principle build cross compilers targeting arbitrary platforms.
+    </para></note>
+    <warning><para>
+      From the above, one would surmise that if a package is being built with a <literal>(build, host, target)</literal> platform triple of <literal>(foo, bar, bar)</literal>, then its build-time dependencies would have a triple of <literal>(foo, foo, bar)</literal>, and <emphasis>those packages'</emphasis> build-time dependencies would have triple of <literal>(foo, foo, foo)</literal>.
+      In other words, it should take two "rounds" of following build-time dependency edges before one reaches a fixed point where, by the sliding window principle, the platform triple no longer changes.
+      Unfortunately, at the moment, we do <emphasis>not</emphasis> implement this correctly, and after only one round of following build-time dependencies is the fixed point reached, with target incorrectly kept different than the others.
+    </para></warning>
+    <para>
+      How does this work in practice? Nixpkgs is now structured so that build-time dependencies are taken from from <varname>buildPackages</varname>, whereas run-time dependencies are taken from the top level attribute set.
+      For example, <varname>buildPackages.gcc</varname> should be used at build time, while <varname>gcc</varname> should be used at run time.
+      Now, for most of Nixpkgs's history, there was no <varname>buildPackages</varname>, and most packages have not been refactored to use it explicitly.
+      Instead, one can use the four attributes used for specifying dependencies as documented in <link linkend="ssec-stdenv-attributes" />.
+      We "splice" together the run-time and build-time package sets with <varname>callPackage</varname>, and then <varname>mkDerivation</varname> for each of four attributes pulls the right derivation out.
+      This splicing can be skipped when not cross compiling as the package sets are the same, but is a bit slow for cross compiling.
+      Because of this, a best-of-both-worlds solution is in the works with no splicing or explicit access of <varname>buildPackages</varname> needed.
+      For now, feel free to use either method.
+    </para>
+  </section>
+
+</section>
+
+<!--============================================================-->
+
+<section xml:id="sec-cross-usage">
+  <title>Cross-building packages</title>
+  <para>
+    To be written.
+    This is basically unchanged so see the old wiki for now.
+  </para>
+</section>
+
+<!--============================================================-->
+
+<section xml:id="sec-cross-infra">
+  <title>Cross-compilation infrastructure</title>
+  <para>To be written.</para>
+  <note><para>
+    If one explores nixpkgs, they will see derivations with names like <literal>gccCross</literal>.
+    Such <literal>*Cross</literal> derivations is a holdover from before we properly distinguished between the host and target platforms
+    —the derivation with "Cross" in the name covered the <literal>build = host != target</literal> case, while the other covered the <literal>host = target</literal>, with build platform the same or not based on whether one was using its <literal>.nativeDrv</literal> or <literal>.crossDrv</literal>.
+    This ugliness will disappear soon.
+  </para></note>
+</section>
+
+</chapter>
diff --git a/doc/manual.xml b/doc/manual.xml
index 1c0dac6e4df..75bd21557fd 100644
--- a/doc/manual.xml
+++ b/doc/manual.xml
@@ -13,6 +13,7 @@
   <xi:include href="quick-start.xml" />
   <xi:include href="stdenv.xml" />
   <xi:include href="multiple-output.xml" />
+  <xi:include href="cross-compilation.xml" />
   <xi:include href="configuration.xml" />
   <xi:include href="functions.xml" />
   <xi:include href="meta.xml" />