[PATCH v4 1/9] KVM: arm64: Add build-time check for duplicate DECLARE_REG use

[Fuad Tabba <tabba@google.com>]

The DECLARE_REG() macro provides a convenient way to create a local
variable initialized from a cpu context in the hyp trap handlers.
However, a common error is to use the macro multiple times in the same
scope with the same register index, but for different logical purposes.

This results in valid C code that compiles without error, but introduces
subtle bugs where a developer expects two different variables to hold
values from two different registers, when in fact they are both sourced
from the same one.

To prevent this entire class of bugs, modify the DECLARE_REG() macro
to declare a dummy variable whose name is derived from the register
index. If the macro is used again with the same index in the same
scope, the compiler will fail with a "redeclaration of variable"
error, turning a subtle runtime bug into an obvious build-time failure.

Signed-off-by: Fuad Tabba <tabba@google.com>
---
Note: I considered a more restrictive compile-time check to also enforce
that the register indexes are sequential and without gaps. However,
achieving this without requiring significant refactoring of all the call
sites results in too much churn. This patch catches most (or all) of the
related issues we have run into in practice.
---
 arch/arm64/kvm/hyp/include/nvhe/trap_handler.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hyp/include/nvhe/trap_handler.h b/arch/arm64/kvm/hyp/include/nvhe/trap_handler.h
index 1e6d995968a1..ba5382c12787 100644
--- a/arch/arm64/kvm/hyp/include/nvhe/trap_handler.h
+++ b/arch/arm64/kvm/hyp/include/nvhe/trap_handler.h
@@ -12,7 +12,8 @@
 #include <asm/kvm_host.h>
 
 #define cpu_reg(ctxt, r)	(ctxt)->regs.regs[r]
-#define DECLARE_REG(type, name, ctxt, reg)	\
+#define DECLARE_REG(type, name, ctxt, reg)					\
+				__always_unused int ___check_reg_ ## reg;	\
 				type name = (type)cpu_reg(ctxt, (reg))
 
 #endif /* __ARM64_KVM_NVHE_TRAP_HANDLER_H__ */
-- 
2.51.0.384.g4c02a37b29-goog