From: Stefan Hajnoczi <stefa...@redhat.com> 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 <stefa...@redhat.com> [DB: split off patch & rewrite guidance on when to use the annotation] Signed-off-by: Daniel P. Berrangé <berra...@redhat.com> --- include/qemu/compiler.h | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h index 496dac5ac1..65b89958d3 100644 --- a/include/qemu/compiler.h +++ b/include/qemu/compiler.h @@ -207,6 +207,26 @@ # 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 + /* * http://clang.llvm.org/docs/ThreadSafetyAnalysis.html * -- 2.49.0
