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-rw-r--r--doc/guix.texi46
-rw-r--r--gnu/services/virtualization.scm21
2 files changed, 64 insertions, 3 deletions
diff --git a/doc/guix.texi b/doc/guix.texi
index f0618e39f0..6245d54e8d 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -5621,11 +5621,26 @@ The following derivations will be built:
Attempt to build for @var{system}---e.g., @code{i686-linux}---instead of
the system type of the build host.
+@quotation Note
+The @code{--system} flag is for @emph{native} compilation and must not
+be confused with cross-compilation. See @code{--target} below for
+information on cross-compilation.
+@end quotation
+
An example use of this is on Linux-based systems, which can emulate
different personalities. For instance, passing
-@code{--system=i686-linux} on an @code{x86_64-linux} system allows users
+@code{--system=i686-linux} on an @code{x86_64-linux} system allows you
to build packages in a complete 32-bit environment.
+Similarly, when transparent emulation with QEMU and @code{binfmt_misc}
+is enabled (@pxref{Virtualization Services,
+@code{qemu-binfmt-service-type}}), you can build for any system for
+which a QEMU @code{binfmt_misc} handler is installed.
+
+Builds for a system other than that of the machine you are using can
+also be offloaded to a remote machine of the right architecture.
+@xref{Daemon Offload Setup}, for more information on offloading.
+
@item --target=@var{triplet}
@cindex cross-compilation
Cross-build for @var{triplet}, which must be a valid GNU triplet, such
@@ -17698,6 +17713,35 @@ This is the configuration for the @code{qemu-binfmt} service.
The list of emulated QEMU platforms. Each item must be a @dfn{platform
object} as returned by @code{lookup-qemu-platforms} (see below).
+@item @code{guix-support?} (default: @code{#f})
+When it is true, QEMU and all its dependencies are added to the build
+environment of @command{guix-daemon} (@pxref{Invoking guix-daemon,
+@code{--chroot-directory} option}). This allows the @code{binfmt_misc}
+handlers to be used within the build environment, which in turn means
+that you can transparently build programs for another architecture.
+
+For example, let's suppose you're on an x86_64 machine and you have this
+service:
+
+@example
+(service qemu-binfmt-service-type
+ (qemu-binfmt-configuration
+ (platforms (lookup-qemu-platforms "arm"))
+ (qemu-support? #t)))
+@end example
+
+You can run:
+
+@example
+guix build -s armhf-linux inkscape
+@end example
+
+@noindent
+and it will build Inkscape for ARMv7 @emph{as if it were a native
+build}, transparently using QEMU to emulate the ARMv7 CPU. Pretty handy
+if you'd like to test a package build for an architecture you don't have
+access to!
+
@item @code{qemu} (default: @code{qemu})
The QEMU package to use.
@end table
diff --git a/gnu/services/virtualization.scm b/gnu/services/virtualization.scm
index 0a8f67fb8e..bf71e7f26a 100644
--- a/gnu/services/virtualization.scm
+++ b/gnu/services/virtualization.scm
@@ -662,7 +662,9 @@ potential infinite waits blocking libvirt."))
(qemu qemu-binfmt-configuration-qemu
(default qemu))
(platforms qemu-binfmt-configuration-platforms
- (default '()))) ;safest default
+ (default '())) ;safest default
+ (guix-support? qemu-binfmt-configuration-guix-support?
+ (default #f)))
(define (qemu-platform->binfmt qemu platform)
"Return a gexp that evaluates to a binfmt string for PLATFORM, using the
@@ -724,6 +726,19 @@ given QEMU package."
'#$(map qemu-platform-name platforms))
#f)))))))
+(define qemu-binfmt-guix-chroot
+ (match-lambda
+ ;; Add QEMU and its dependencies to the guix-daemon chroot so that our
+ ;; binfmt_misc handlers work in the chroot (otherwise 'execve' would fail
+ ;; with ENOENT.)
+ ;;
+ ;; The 'F' flag of binfmt_misc is meant to address this problem by loading
+ ;; the interpreter upfront rather than lazily, but apparently that is
+ ;; insufficient (perhaps it loads the 'qemu-ARCH' binary upfront but looks
+ ;; up its dependencies lazily?).
+ (($ <qemu-binfmt-configuration> qemu platforms guix?)
+ (if guix? (list qemu) '()))))
+
(define qemu-binfmt-service-type
;; TODO: Make a separate binfmt_misc service out of this?
(service-type (name 'qemu-binfmt)
@@ -732,7 +747,9 @@ given QEMU package."
(const
(list %binary-format-file-system)))
(service-extension shepherd-root-service-type
- qemu-binfmt-shepherd-services)))
+ qemu-binfmt-shepherd-services)
+ (service-extension guix-service-type
+ qemu-binfmt-guix-chroot)))
(default-value (qemu-binfmt-configuration))
(description
"This service supports transparent emulation of binaries