cross-compilation for a missing target with guix build

for example, i can build for the target mips64el-linux-gnu:

$ guix build --no-substitutes --target=mips64el-linux-gnu hello
...
successfully built /gnu/store/sdhhi1fhgbldl5qyfwvwlzhf2gq41n0j-hello-2.10.drv
/gnu/store/rbl4yrjyj3vxn27cap1y24z5wkr90p8y-hello-2.10

but not for the target mipsel-linux-gnu:

$ guix build --no-substitutes --target=mipsel-linux-gnu hello
Backtrace:
In guix/store.scm:
   1341:2 19 (map/accumulate-builds #<store-connection 256.99 7f2ad…> …)
In srfi/srfi-1.scm:
   586:17 18 (map1 (#<package [email protected] gnu/packages/base.scm:74 7…>))
In guix/store.scm:
   1300:8 17 (call-with-build-handler #<procedure build-accumulator…> …)
In guix/scripts/build.scm:
   576:18 16 (_ _)
In guix/packages.scm:
  1169:16 15 (package-cross-derivation _ #<package [email protected] gnu/p…> …)
  1510:22 14 (thunk)
  1447:22 13 (bag->cross-derivation #<store-connection 256.99 7f2ad…> …)
In srfi/srfi-1.scm:
   586:17 12 (map1 (("cross-gcc" #<package gcc-cross-mipsel-lin…>) …))
In guix/packages.scm:
  1212:16 11 (expand-input #<store-connection 256.99 7f2ad8295910> #f …)
  1169:16 10 (package-derivation _ #<package gcc-cross-mipsel-linux…> …)
  1486:22  9 (thunk)
  1169:16  8 (package->bag _ _ _ #:graft? _)
  1279:43  7 (thunk)
In gnu/packages/cross-base.scm:
    265:9  6 (arguments #<package [email protected] g…>)
   121:20  5 (cross-gcc-arguments "mipsel-linux-gnu" #<package gcc@…> …)
In gnu/packages/gcc.scm:
   235:52  4 (arguments #<package [email protected] gnu/packages/gcc.scm:364…>)
In gnu/packages/bootstrap.scm:
   306:14  3 (glibc-dynamic-linker _)
In ice-9/boot-9.scm:
  1669:16  2 (raise-exception _ #:continuable? _)
  1669:16  1 (raise-exception _ #:continuable? _)
  1669:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1669:16: In procedure raise-exception:
dynamic linker name not known for this system "mipsel-linux"

how to build for this target? or at least find out the full list of available targets?

Solution

find out the full list of available targets

all active targets are listed in the glibc-dynamic-linker function in gnu/packages/bootstrap.scm.

how to build for this target?

you can:

create an empty directory with your future packages;
save the path to it into the environment variable $GUIX_PACKAGE_PATH;
place the mentioned bootstrap.scm file here: $GUIX_PACKAGE_PATH/gnu/packages/bootstrap.scm;

make the necessary changes to it:

diff --git a/gnu/packages/bootstrap.scm b/gnu/packages/bootstrap.scm
index 5a8028a465..102e7378ae 100644
--- a/gnu/packages/bootstrap.scm
+++ b/gnu/packages/bootstrap.scm
@@ -317,6 +317,7 @@ or false to signal an error."
         ((string=? system "alpha-linux") "/lib/ld-linux.so.2")
         ((string=? system "s390x-linux") "/lib/ld64.so.1")
         ((string=? system "riscv64-linux") "/lib/ld-linux-riscv64-lp64d.so.1")
+        ((string=? system "mipsel-linux") "/lib/ld.so.1")

         ;; XXX: This one is used bare-bones, without a libc, so add a case
         ;; here just so we can keep going.

and build a package (at the same time, a complete set of compiler/binutils/etc. - gcc-toolchain - will be built):
```
$ guix build --target=mipsel-linux-gnu hello
...
/gnu/store/2vcvqn3c4ngif1l6s2dxambcpdmyywgc-hello-2.10
```