Re: [PATCH v2 03/13] KVM: selftests: Fudge around an apparent gcc bug in arm64's PMU test

Sean Christopherson
2024-09-30T22:48:40Z

On Mon, Sep 30, 2024, Sean Christopherson wrote:
> On Wed, Sep 11, 2024, Sean Christopherson wrote:
> > Use u64_replace_bits() instead of u64p_replace_bits() to set PMCR.N in
> > arm64's vPMU counter access test to fudge around what appears to be a gcc
> > bug. With the recent change to have vcpu_get_reg() return a value in lieu
> > of an out-param, some versions of gcc completely ignore the operation
> > performed by set_pmcr_n(), i.e. ignore the output param.
>
> Filed a gcc bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116912
>
> I'll report back if anything interesting comes out of that bug.

Well, there goes several hours that I'll never get back. Selftests are compiled
with -O2, which enables strict-aliasing optimizations, and "unsigned long" and
"unsigned long long" technically don't alias despite being the same size on 64-bit
builds, so the compiler is allowed to optimize away the load. *sigh*

I'll replace this with a patch to disable strict-aliasing, which the kernel has
done since forever (literally predates git). Grr.

diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
index 48d32c5aa3eb..a6f92129bb02 100644
--- a/tools/testing/selftests/kvm/Makefile
+++ b/tools/testing/selftests/kvm/Makefile
@@ -235,10 +235,10 @@ CFLAGS += -Wall -Wstrict-prototypes -Wuninitialized -O2 -g -std=gnu99 \
-Wno-gnu-variable-sized-type-not-at-end -MD -MP -DCONFIG_64BIT \
-fno-builtin-memcmp -fno-builtin-memcpy \
-fno-builtin-memset -fno-builtin-strnlen \
- -fno-stack-protector -fno-PIE -I$(LINUX_TOOL_INCLUDE) \
- -I$(LINUX_TOOL_ARCH_INCLUDE) -I$(LINUX_HDR_PATH) -Iinclude \
- -I$(<D) -Iinclude/$(ARCH_DIR) -I ../rseq -I.. $(EXTRA_CFLAGS) \
- $(KHDR_INCLUDES)
+ -fno-stack-protector -fno-PIE -fno-strict-aliasing \
+ -I$(LINUX_TOOL_INCLUDE) -I$(LINUX_TOOL_ARCH_INCLUDE) \
+ -I$(LINUX_HDR_PATH) -Iinclude -I$(<D) -Iinclude/$(ARCH_DIR) \
+ -I ../rseq -I.. $(EXTRA_CFLAGS) $(KHDR_INCLUDES)