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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Aug 04, 2023 at 06:19:54PM -0500, Josh Poimboeuf wrote:

> Looks mostly right, except this now creates an unnecessary gap in
> unwinding coverage for the ORC unwinder.  So it's better to put the
> FP-specific changes behind CONFIG_FRAME_POINTER:

Fair enough.

> 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..51f6851b1ae5 100644
> --- a/arch/x86/kvm/svm/vmenter.S
> +++ b/arch/x86/kvm/svm/vmenter.S
> @@ -99,6 +99,9 @@
>   */
>  SYM_FUNC_START(__svm_vcpu_run)
>  	push %_ASM_BP
> +#ifdef CONFIG_FRAME_POINTER
> +	mov %_ASM_SP, %_ASM_BP
> +#endif
>  #ifdef CONFIG_X86_64
>  	push %r15
>  	push %r14
> @@ -121,7 +124,20 @@ SYM_FUNC_START(__svm_vcpu_run)
>  	/* Needed to restore access to percpu variables.  */
>  	__ASM_SIZE(push) PER_CPU_VAR(svm_data + SD_save_area_pa)
>  
> -	/* Finally save @svm. */
> +	/*
> +	 * Finally save frame pointer and @svm.
> +	 *
> +	 * Clobbering BP here is mostly ok since GIF will block NMIs and with
> +	 * the exception of #MC and the kvm_rebooting _ASM_EXTABLE()s below
> +	 * nothing untoward will happen until BP is restored.
> +	 *
> +	 * The kvm_rebooting exceptions should not want to unwind stack, and
> +	 * while #MV might want to unwind stack, it is ultimately fatal.
> +	 */

Aside from me not being able to type #MC, I did realize that the
kvm_reboot exception will go outside noinstr code and can hit
tracing/instrumentation and do unwinds from there.