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

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Aug 10, 2023 at 04:17:41PM +0200, Paolo Bonzini wrote:
> On 8/5/23 02:55, Peter Zijlstra wrote:
> > > +	 * 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.
> 
> Asynchronously disabling SVM requires an IPI, so kvm_rebooting cannot change
> within CLGI/STGI.   We can check it after CLGI instead of waiting for a #GP:

Seems fair; thanks!

> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 956726d867aa..e3755f5eaf81 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4074,7 +4074,10 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct
> kvm_vcpu *vcpu)
>  	if (!static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
>  		x86_spec_ctrl_set_guest(svm->virt_spec_ctrl);
> 
> -	svm_vcpu_enter_exit(vcpu, spec_ctrl_intercepted);
> +	if (unlikely(kvm_rebooting))
> +		svm->vmcb->control.exit_code = SVM_EXIT_PAUSE;
> +	else
> +		svm_vcpu_enter_exit(vcpu, spec_ctrl_intercepted);
> 
>  	if (!static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
>  		x86_spec_ctrl_restore_host(svm->virt_spec_ctrl);
> diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S
> index 8e8295e774f0..34641b3a6823 100644
> --- a/arch/x86/kvm/svm/vmenter.S
> +++ b/arch/x86/kvm/svm/vmenter.S
> @@ -270,23 +270,12 @@ SYM_FUNC_START(__svm_vcpu_run)
>  	RESTORE_GUEST_SPEC_CTRL_BODY
>  	RESTORE_HOST_SPEC_CTRL_BODY
> 
> -10:	cmpb $0, kvm_rebooting
> -	jne 2b
> -	ud2
> -30:	cmpb $0, kvm_rebooting
> -	jne 4b
> -	ud2
> -50:	cmpb $0, kvm_rebooting
> -	jne 6b
> -	ud2
> -70:	cmpb $0, kvm_rebooting
> -	jne 8b
> -	ud2
> +10:	ud2
> 
>  	_ASM_EXTABLE(1b, 10b)
> -	_ASM_EXTABLE(3b, 30b)
> -	_ASM_EXTABLE(5b, 50b)
> -	_ASM_EXTABLE(7b, 70b)
> +	_ASM_EXTABLE(3b, 10b)
> +	_ASM_EXTABLE(5b, 10b)
> +	_ASM_EXTABLE(7b, 10b)
> 
>  SYM_FUNC_END(__svm_vcpu_run)
> 
> 
> Paolo
>