Re: [PATCH 2/3] x86/split_lock: Refactor and export handle_user_split_lock() for KVM

[Xiaoyao Li <xiaoyao.li@xxxxxxxxx>]

On 4/3/2020 3:06 AM, Thomas Gleixner wrote:
> Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:
> > On Thu, Apr 02, 2020 at 07:01:56PM +0200, Thomas Gleixner wrote:
> > > >   static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
> > > >   static inline void switch_to_sld(unsigned long tifn) {}
> > > > -static inline bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> > > > +static inline bool handle_user_split_lock(unsigned long ip)
> > >
> > > This is necessary because VMX can be compiled without CPU_SUP_INTEL?
> >
> > Ya, it came about when cleaning up the IA32_FEATURE_CONTROL MSR handling
> > to consolidate duplicate code.
> >
> > config KVM_INTEL
> >          tristate "KVM for Intel (and compatible) processors support"
> >          depends on KVM && IA32_FEAT_CTL
> >
> > config IA32_FEAT_CTL
> >          def_bool y
> >          depends on CPU_SUP_INTEL || CPU_SUP_CENTAUR || CPU_SUP_ZHAOXIN
>
> Ah, indeed. So something like the below would make sense. Hmm?
>
> Of course that can be mangled into Xiaoyao's patches, I'm not worried
> about my patch count :)

I don't mind using yours in my next version.

Hi Paolo,

Are you OK with the kvm part below?

If no objection, I can spin the next version using tglx's.

> 8<----------------
> --- a/arch/x86/include/asm/cpu.h
> +++ b/arch/x86/include/asm/cpu.h
> @@ -43,14 +43,14 @@ unsigned int x86_stepping(unsigned int s
>   #ifdef CONFIG_CPU_SUP_INTEL
>   extern void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c);
>   extern void switch_to_sld(unsigned long tifn);
> -extern bool handle_user_split_lock(struct pt_regs *regs, long error_code);
> +extern int handle_ac_split_lock(unsigned long ip);
>   extern void split_lock_validate_module_text(struct module *me, void *text, void *text_end);
>   #else
>   static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
>   static inline void switch_to_sld(unsigned long tifn) {}
> -static inline bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> +static int handle_ac_split_lock(unsigned long ip)
>   {
> -	return false;
> +	return -ENOSYS;
>   }
>   static inline void split_lock_validate_module_text(struct module *me, void *text, void *text_end) {}
>   #endif
>
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -1102,13 +1102,20 @@ static void split_lock_init(void)
>   	split_lock_verify_msr(sld_state != sld_off);
>   }
>   
> -bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> +int handle_ac_split_lock(unsigned long ip)
>   {
> -	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
> -		return false;
> +	switch (sld_state) {
> +	case sld_warn:
> +		break;
> +	case sld_off:
> +		pr_warn_once("#AC: Spurious trap at address: 0x%lx\n", ip);
> +		return -ENOSYS;
> +	case sld_fatal:
> +		return -EFAULT;
> +	}
>   
>   	pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
> -			    current->comm, current->pid, regs->ip);
> +			    current->comm, current->pid, ip);
>   
>   	/*
>   	 * Disable the split lock detection for this task so it can make
> @@ -1117,8 +1124,9 @@ bool handle_user_split_lock(struct pt_re
>   	 */
>   	sld_update_msr(false);
>   	set_tsk_thread_flag(current, TIF_SLD);
> -	return true;
> +	return 0;
>   }
> +EXPORT_SYMBOL_GPL(handle_ac_split_lock);
>   
>   /*
>    * This function is called only when switching between tasks with
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -304,7 +304,7 @@ dotraplinkage void do_alignment_check(st
>   
>   	local_irq_enable();
>   
> -	if (handle_user_split_lock(regs, error_code))
> +	if (!(regs->flags & X86_EFLAGS_AC) && !handle_ac_split_lock(regs->ip))
>   		return;
>   
>   	do_trap(X86_TRAP_AC, SIGBUS, "alignment check", regs,
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -65,6 +65,7 @@
>   
>   MODULE_AUTHOR("Qumranet");
>   MODULE_LICENSE("GPL");
> +MODULE_INFO(sld_safe, "Y");
>   
>   #ifdef MODULE
>   static const struct x86_cpu_id vmx_cpu_id[] = {
> @@ -4623,6 +4624,22 @@ static int handle_machine_check(struct k
>   	return 1;
>   }
>   
> +static bool guest_handles_ac(struct kvm_vcpu *vcpu)
> +{
> +	/*
> +	 * If guest has alignment checking enabled in CR0 and activated in
> +	 * eflags, then the #AC originated from CPL3 and the guest is able
> +	 * to handle it. It does not matter whether this is a regular or
> +	 * a split lock operation induced #AC.
> +	 */
> +	if (vmx_get_cpl(vcpu) == 3 && kvm_read_cr0_bits(vcpu, X86_CR0_AM) &&
> +	    kvm_get_rflags(vcpu) & X86_EFLAGS_AC)
> +		return true;
> +
> +	/* Add guest SLD handling checks here once it's supported */
> +	return false;
> +}
> +
>   static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>   {
>   	struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -4630,6 +4647,7 @@ static int handle_exception_nmi(struct k
>   	u32 intr_info, ex_no, error_code;
>   	unsigned long cr2, rip, dr6;
>   	u32 vect_info;
> +	int err;
>   
>   	vect_info = vmx->idt_vectoring_info;
>   	intr_info = vmx->exit_intr_info;
> @@ -4688,9 +4706,6 @@ static int handle_exception_nmi(struct k
>   		return handle_rmode_exception(vcpu, ex_no, error_code);
>   
>   	switch (ex_no) {
> -	case AC_VECTOR:
> -		kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
> -		return 1;
>   	case DB_VECTOR:
>   		dr6 = vmcs_readl(EXIT_QUALIFICATION);
>   		if (!(vcpu->guest_debug &
> @@ -4719,6 +4734,29 @@ static int handle_exception_nmi(struct k
>   		kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip;
>   		kvm_run->debug.arch.exception = ex_no;
>   		break;
> +	case AC_VECTOR:
> +		if (guest_handles_ac(vcpu)) {
> +			kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
> +			return 1;
> +		}
> +		/*
> +		 * Handle #AC caused by split lock detection. If the host
> +		 * mode is sld_warn, then it warns, marks current with
> +		 * TIF_SLD and disables split lock detection. So the guest
> +		 * can just continue.
> +		 *
> +		 * If the host mode is fatal, the handling code warned. Let
> +		 * qemu kill itself.
> +		 *
> +		 * If the host mode is off, then this #AC is bonkers and
> +		 * something is badly wrong. Let it fail as well.
> +		 */
> +		err = handle_ac_split_lock(kvm_rip_read(vcpu));
> +		if (!err)
> +			return 1;
> +		/* Propagate the error type to user space */
> +		error_code = err == -EFAULT ? 0x100 : 0x200;
> +		fallthrough;
>   	default:
>   		kvm_run->exit_reason = KVM_EXIT_EXCEPTION;
>   		kvm_run->ex.exception = ex_no;