[PATCH 2/3] avocado_qemu: add AVOCADO_DEFAULT_ARCH for cross-arch tests

[Daniel Henrique Barboza]

All avocado tests that are arch agnostic (i.e. does not set an 'arch'
tag) are run with arch=None in pick_default_qemu_bin(), and then 'arch'
is set to os.uname()[4], meaning that it will take the arch of the
running host.

This means that if one compiles QEMU binaries for non-x86 targets on an
x86 machine, and then run 'make check-avocado', all arch agnostic tests
will be cancelled because there's no qemu-system-x86_64 to be found.

There is no particular reason to not allow these tests to be run with
other arch binaries in a x86_64 host. Allow the developer to do it by
adding a a new env variable called AVOCADO_DEFAULT_ARCH. Any 'arch' that
is set by this variable will take precedence of setting it via
os.uname()[4]. We can then run non-x86 binaries tests in a x86_64 host
as follows:

$ AVOCADO_DEFAULT_ARCH=riscv64 make check-avocado
(...)
RESULTS: PASS 11 | ERROR 0 | FAIL 0 | SKIP 1 | WARN 0 | INTERRUPT 0 | CANCEL 0

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
---
 docs/devel/testing.rst                 | 7 +++++++
 tests/avocado/avocado_qemu/__init__.py | 9 ++++++---
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/docs/devel/testing.rst b/docs/devel/testing.rst
index e10c47b5a7..95d0a3e626 100644
--- a/docs/devel/testing.rst
+++ b/docs/devel/testing.rst
@@ -1144,6 +1144,13 @@ the framework or by the test itself.  At the framework 
level, it will
 currently influence the selection of a QEMU binary (when one is not
 explicitly given).
 
+When ``arch`` is not set, an env variable AVOCADO_DEFAULT_ARCH can
+be used as default value if set.  This allows hosts of different
+architectures to run arch-agnostic tests using binaries from other
+archs (i.e. a x86_64 host can use aarch64/riscv64 binaries as
+default).  If this variable isn't set, ``arch`` defaults to the
+host system arch given by ``os.uname``.
+
 Tests are also free to use this attribute value, for their own needs.
 A test may, for instance, use the same value when selecting the
 architecture of a kernel or disk image to boot a VM with.
diff --git a/tests/avocado/avocado_qemu/__init__.py 
b/tests/avocado/avocado_qemu/__init__.py
index 8614ac3978..bc42985cbb 100644
--- a/tests/avocado/avocado_qemu/__init__.py
+++ b/tests/avocado/avocado_qemu/__init__.py
@@ -107,14 +107,17 @@ def pick_default_qemu_bin(bin_prefix='qemu-system-', 
arch=None):
     directory or in the source tree root directory.
 
     :param arch: the arch to use when looking for a QEMU binary (the target
-                 will match the arch given).  If None (the default), arch
-                 will be the current host system arch (as given by
-                 :func:`os.uname`).
+                 will match the arch given).  If None (the default), check
+                 if the AVOCADO_DEFAULT_ARCH env var is set and use it as
+                 arch.  If it's not set, arch will be the current host
+                 system arch (as given by :func:`os.uname`).
     :type arch: str
     :returns: the path to the default QEMU binary or None if one could not
               be found
     :rtype: str or None
     """
+    if arch is None:
+        arch = os.getenv('AVOCADO_DEFAULT_ARCH')
     if arch is None:
         arch = os.uname()[4]
     # qemu binary path does not match arch for powerpc, handle it
-- 
2.39.0