169 lines
13 KiB
XML
169 lines
13 KiB
XML
<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>Packaging 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>
|
|
<para>
|
|
Some examples will probably make this clearer.
|
|
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.
|
|
Indeed, this happens with cross compilation, where only rounds of native dependencies starting with the second necessarily coincide with native packages.
|
|
</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>
|
|
<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>
|
|
<note><para>
|
|
More information needs to moved from the old wiki, especially <link xlink:href="https://nixos.org/wiki/CrossCompiling" />, for this section.
|
|
</para></note>
|
|
<para>
|
|
Many sources (manual, wiki, etc) probably mention passing <varname>system</varname>, <varname>platform</varname>, and, optionally, <varname>crossSystem</varname> to nixpkgs:
|
|
<literal>import <nixpkgs> { system = ..; platform = ..; crossSystem = ..; }</literal>.
|
|
<varname>system</varname> and <varname>platform</varname> together determine the system on which packages are built, and <varname>crossSystem</varname> specifies the platform on which packages are ultimately intended to run, if it is different.
|
|
This still works, but with more recent changes, one can alternatively pass <varname>localSystem</varname>, containing <varname>system</varname> and <varname>platform</varname>, for symmetry.
|
|
</para>
|
|
<para>
|
|
One would think that <varname>localSystem</varname> and <varname>crossSystem</varname> overlap horribly with the three <varname>*Platforms</varname> (<varname>buildPlatform</varname>, <varname>hostPlatform,</varname> and <varname>targetPlatform</varname>; see <varname>stage.nix</varname> or the manual).
|
|
Actually, those identifiers are purposefully not used here to draw a subtle but important distinction:
|
|
While the granularity of having 3 platforms is necessary to properly *build* packages, it is overkill for specifying the user's *intent* when making a build plan or package set.
|
|
A simple "build vs deploy" dichotomy is adequate: the sliding window principle described in the previous section shows how to interpolate between the these two "end points" to get the 3 platform triple for each bootstrapping stage.
|
|
That means for any package a given package set, even those not bound on the top level but only reachable via dependencies or <varname>buildPackages</varname>, the three platforms will be defined as one of <varname>localSystem</varname> or <varname>crossSystem</varname>, with the former replacing the latter as one traverses build-time dependencies.
|
|
A last simple difference then is <varname>crossSystem</varname> should be null when one doesn't want to cross-compile, while the <varname>*Platform</varname>s are always non-null.
|
|
<varname>localSystem</varname> is always non-null.
|
|
</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>
|