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.

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-lkp@intel.com/
Link: https://gist.github.com/ba710e3703bf45043a31e2806c843ffd
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 arch/powerpc/kvm/book3s_hv_nested.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)


base-commit: 4a21192e2796c3338c4b0083b494a84a61311aaf

Excerpts from Nathan Chancellor's message of June 22, 2021 4:24 am:
> 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-lkp@intel.com/
> Link: https://gist.github.com/ba710e3703bf45043a31e2806c843ffd
> Reported-by: kernel test robot <lkp@intel.com>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Seems okay to me. If it was something where performance might be 
signficiant I guess you could ifdef on CC_IS_CLANG, but for this
it shouldn't matter.

Acked-by: Nicholas Piggin <npiggin@gmail.com>

Thanks,
Nick

> ---
>  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
> 
>

On Mon, 21 Jun 2021 11:24:40 -0700, Nathan Chancellor wrote:
> 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.
> 
> [...]

Applied to powerpc/topic/ppc-kvm.

[1/1] KVM: PPC: Book3S HV: Workaround high stack usage with clang
      https://git.kernel.org/powerpc/c/51696f39cbee5bb684e7959c0c98b5f54548aa34

cheers