LLVM does not emit optimal byteswap assembly, which results in high
stack usage in kvmhv_enter_nested_guest() due to the inlining of
byteswap_pt_regs(). With LLVM 12.0.0:
arch/powerpc/kvm/book3s_hv_nested.c:289:6: error: stack frame size of
2512 bytes in function 'kvmhv_enter_nested_guest' [-Werror,-Wframe-larger-than=]
long kvmhv_enter_nested_guest(struct kvm_vcpu *vcpu)
^
1 error generated.
While this gets fixed in LLVM, mark byteswap_pt_regs() as
noinline_for_stack so that it does not get inlined and break the build
due to -Werror by default in arch/powerpc/. Not inlining saves
approximately 800 bytes with LLVM 12.0.0:
arch/powerpc/kvm/book3s_hv_nested.c:290:6: warning: stack frame size of
1728 bytes in function 'kvmhv_enter_nested_guest' [-Wframe-larger-than=]
long kvmhv_enter_nested_guest(struct kvm_vcpu *vcpu)
^
1 warning generated.
Link: https://github.com/ClangBuiltLinux/linux/issues/1292
Link: https://bugs.llvm.org/show_bug.cgi?id=49610
Link: https://lore.kernel.org/r/202104031853.vdt0qjqj-...@intel.com/
Link: https://gist.github.com/ba710e3703bf45043a31e2806c843ffd
Reported-by: kernel test robot <l...@intel.com>
Signed-off-by: Nathan Chancellor <nat...@kernel.org>
---
arch/powerpc/kvm/book3s_hv_nested.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/arch/powerpc/kvm/book3s_hv_nested.c
b/arch/powerpc/kvm/book3s_hv_nested.c
index 60724f674421..1b3ff0af1264 100644
--- a/arch/powerpc/kvm/book3s_hv_nested.c
+++ b/arch/powerpc/kvm/book3s_hv_nested.c
@@ -53,7 +53,8 @@ void kvmhv_save_hv_regs(struct kvm_vcpu *vcpu, struct
hv_guest_state *hr)
hr->dawrx1 = vcpu->arch.dawrx1;
}
-static void byteswap_pt_regs(struct pt_regs *regs)
+/* Use noinline_for_stack due to https://bugs.llvm.org/show_bug.cgi?id=49610 */
+static noinline_for_stack void byteswap_pt_regs(struct pt_regs *regs)
{
unsigned long *addr = (unsigned long *) regs;
base-commit: 4a21192e2796c3338c4b0083b494a84a61311aaf
--
2.32.0.93.g670b81a890
