KVM: PPC: Book3S HV: Workaround high stack usage with clang

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.

Cc: [email protected] # v4.20+
Reported-by: kernel test robot <[email protected]>
Signed-off-by: Nathan Chancellor <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: ClangBuiltLinux/linux#1292
Link: https://bugs.llvm.org/show_bug.cgi?id=49610
Link: https://lore.kernel.org/r/[email protected]/
Link: https://gist.github.com/ba710e3703bf45043a31e2806c843ffd
Link: https://lore.kernel.org/r/[email protected]

Author: nathanchance

arch/powerpc/kvm/book3s_hv_nested.c

@@ -54,7 +54,8 @@ 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;