qemu-kvm/SOURCES/kvm-include-qemu-compiler-add-QEMU_UNINITIALIZED-attribu.patch

81 lines
3.2 KiB
Diff

From 73f85b945f09ae118f2c1479110f2e34906e084b Mon Sep 17 00:00:00 2001
From: Stefan Hajnoczi <stefanha@redhat.com>
Date: Tue, 10 Jun 2025 13:36:39 +0100
Subject: [PATCH 02/31] include/qemu/compiler: add QEMU_UNINITIALIZED attribute
macro
MIME-Version: 1.0
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RH-Author: Stefan Hajnoczi <stefanha@redhat.com>
RH-MergeRequest: 461: Solve -ftrivial-auto-var-init performance regression with QEMU_UNINITIALIZED
RH-Jira: RHEL-99887
RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>
RH-Commit: [1/30] 6b6151625fdf6636cbf352731906f643e2fbfd35
The QEMU_UNINITIALIZED macro is to be used to skip the default compiler
variable initialization done by -ftrivial-auto-var-init=zero.
Use this in cases where there a method in the device I/O path (or other
important hot paths), that has large variables on the stack. A rule of
thumb is that "large" means a method with 4kb data in the local stack
frame. Any variables which are KB in size, should be annotated with this
attribute, to pre-emptively eliminate any potential overhead from the
compiler zero'ing memory.
Given that this turns off a security hardening feature, when using this
to flag variables, it is important that the code is double-checked to
ensure there is no possible use of uninitialized data in the method.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20250610123709.835102-2-berrange@redhat.com
[DB: split off patch & rewrite guidance on when to use the annotation]
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit c653b67d1863b7ebfa67f7c9f4aec209d7b5ced5)
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Conflicts:
include/qemu/compiler.h
Context conflict due to clang Thread Safety Analysis macros.
---
include/qemu/compiler.h | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)
diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
index c06954ccb4..cc193d5b82 100644
--- a/include/qemu/compiler.h
+++ b/include/qemu/compiler.h
@@ -212,6 +212,26 @@
# define QEMU_USED
#endif
+/*
+ * Disable -ftrivial-auto-var-init on a local variable.
+ *
+ * Use this in cases where there a method in the device I/O path (or other
+ * important hot paths), that has large variables on the stack. A rule of
+ * thumb is that "large" means a method with 4kb data in the local stack
+ * frame. Any variables which are KB in size, should be annotated with this
+ * attribute, to pre-emptively eliminate any potential overhead from the
+ * compiler's implicit zero'ing of memory.
+ *
+ * Given that this turns off a security hardening feature, when using this
+ * to flag variables, it is important that the code is double-checked to
+ * ensure there is no possible use of uninitialized data in the method.
+ */
+#if __has_attribute(uninitialized)
+# define QEMU_UNINITIALIZED __attribute__((uninitialized))
+#else
+# define QEMU_UNINITIALIZED
+#endif
+
/*
* Ugly CPP trick that is like "defined FOO", but also works in C
* code. Useful to replace #ifdef with "if" statements; assumes
--
2.39.3