Re: [PATCH] KVM: SVM: Add exception to disable objtool warning for kvm-amd.o

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Aug 02, 2023 at 02:41:07PM +0530, Nikunj A Dadhania wrote:
> commit 7f4b5cde2409 ("kvm: Disable objtool frame pointer checking for
> vmenter.S") had added the vmenter.o file to the exception list.
> 
> objtool gives the following warnings in the newer kernel builds:
> 
>   arch/x86/kvm/kvm-amd.o: warning: objtool: __svm_vcpu_run+0x17d: BP used as a scratch register
>   arch/x86/kvm/kvm-amd.o: warning: objtool: __svm_sev_es_vcpu_run+0x72: BP used as a scratch register
> 
> As kvm-amd.o is a link time object, skipping the kvm-amd.o is not possible
> as per the objtool documentation, better to skip the offending functions.
> 
> Functions __svm_vcpu_run() and __svm_sev_es_vcpu_run() saves and restores
> RBP. Below is the snippet:
> 
>     SYM_FUNC_START(__svm_vcpu_run)
>         push %_ASM_BP
>     <…>
>         pop %_ASM_BP
>         RET
> 
> Add exceptions to skip both these functions. Remove the
> OBJECT_FILES_NON_STANDARD for vmenter.o
> 
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Sean Christopherson <seanjc@google.com>
> Reported-by: Ravi Bangoria <ravi.bangoria@amd.com>
> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
> ---
>  arch/x86/kvm/Makefile      | 4 ----
>  arch/x86/kvm/svm/vmenter.S | 2 ++
>  2 files changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/Makefile b/arch/x86/kvm/Makefile
> index 80e3fe184d17..0c5c2f090e93 100644
> --- a/arch/x86/kvm/Makefile
> +++ b/arch/x86/kvm/Makefile
> @@ -3,10 +3,6 @@
>  ccflags-y += -I $(srctree)/arch/x86/kvm
>  ccflags-$(CONFIG_KVM_WERROR) += -Werror
>  
> -ifeq ($(CONFIG_FRAME_POINTER),y)
> -OBJECT_FILES_NON_STANDARD_vmenter.o := y
> -endif
> -
>  include $(srctree)/virt/kvm/Makefile.kvm
>  
>  kvm-y			+= x86.o emulate.o i8259.o irq.o lapic.o \
> diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S
> index 8e8295e774f0..8fd37d661c33 100644
> --- a/arch/x86/kvm/svm/vmenter.S
> +++ b/arch/x86/kvm/svm/vmenter.S
> @@ -289,6 +289,7 @@ SYM_FUNC_START(__svm_vcpu_run)
>  	_ASM_EXTABLE(7b, 70b)
>  
>  SYM_FUNC_END(__svm_vcpu_run)
> +STACK_FRAME_NON_STANDARD(__svm_vcpu_run)
>  
>  /**
>   * __svm_sev_es_vcpu_run - Run a SEV-ES vCPU via a transition to SVM guest mode
> @@ -388,3 +389,4 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run)
>  	_ASM_EXTABLE(1b, 3b)
>  
>  SYM_FUNC_END(__svm_sev_es_vcpu_run)
> +STACK_FRAME_NON_STANDARD_FP(__svm_sev_es_vcpu_run)

Urgh... no, no, this is all broken.

By marking them with STACK_FRAME_NON_STANDARD you will get no ORC data
at all, and then you also violate the normal framepointer calling
convention.

This means that if you need to unwind here you're up a creek without no
paddles on.

Objtool complains for a reason, your changelog does not provide a
counter argument for that reason.

Hardware/firmware interfaces that require one to violate basic
calling conventions are horrible crap.