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[XEN PATCH 2/2] Address violation of MISRA C Rule 13.1 involving asm side effects.
- To: xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
- Date: Fri, 8 Aug 2025 23:40:47 +0200
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- Cc: sstabellini@xxxxxxxxxx, consulting@xxxxxxxxxxx, dmytro_prokopchuk1@xxxxxxxx, andrew.cooper3@xxxxxxxxxx, jbeulich@xxxxxxxx, Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>, Doug Goldstein <cardoe@xxxxxxxxxx>
- Delivery-date: Fri, 08 Aug 2025 21:41:09 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
The rule states: "Initializer lists shall not contain persistent side effects".
The specific way in which the 'mrs' instruction is used does not lead to
visible side effects for the surrounding code.
Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
---
Not yet tested on the Xen ECLAIR runner, as the syntax used in the deviation
is only supported after updating the runner.
What the tool is reporting is that due to the '=r' constraint and the
semantics of the instruction, there is the side effect of writing to '_r',
but this is not observable outside the stmt expr. The deviation ends up being
a bit too general for my taste, but the restriction on the actual istruction
should be enough to limit applicability to cases that are arguably safe in
practice.
An alternative approach would be represented by stating that side effects in
'READ_SYSREG64' are safe, but this is not true in general.
---
automation/eclair_analysis/ECLAIR/deviations.ecl | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/automation/eclair_analysis/ECLAIR/deviations.ecl
b/automation/eclair_analysis/ECLAIR/deviations.ecl
index ec0cac797e5f..6b492e38505d 100644
--- a/automation/eclair_analysis/ECLAIR/deviations.ecl
+++ b/automation/eclair_analysis/ECLAIR/deviations.ecl
@@ -437,6 +437,10 @@ write or not"
# Series 13
#
+-doc_begin="Consider the asm instruction to read an Arm system register to
have no side effects."
+-asm_properties+={"asm(any())&&child(text, ast_field(value,^mrs\\s+%0.*$))",
{no_side_effect}}
+-doc_end
+
-doc_begin="All developers and reviewers can be safely assumed to be well aware
of the short-circuit evaluation strategy of such logical operators."
-config=MC3A2.R13.5,reports+={disapplied,"any()"}
--
2.43.0
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