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    Subject[PATCH 5.13 075/800] KVM: PPC: Book3S HV: Workaround high stack usage with clang
    Date
    From: Nathan Chancellor <nathan@kernel.org>

    commit 51696f39cbee5bb684e7959c0c98b5f54548aa34 upstream.

    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: stable@vger.kernel.org # v4.20+
    Reported-by: kernel test robot <lkp@intel.com>
    Signed-off-by: Nathan Chancellor <nathan@kernel.org>
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    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
    Link: https://lore.kernel.org/r/20210621182440.990242-1-nathan@kernel.org
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/powerpc/kvm/book3s_hv_nested.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    --- 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
    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;


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