Subject: Re: [PATCH v4 2/2] KVM: PPC: Exit guest upon MCE when FWNMI capability is enabled

From: Balbir Singh <[email-masked]>
Date: Thu, 12 Jan 2017 14:35:53 +0530

On Mon, Jan 09, 2017 at 05:10:45PM +0530, Aravinda Prasad wrote:
> Enhance KVM to cause a guest exit with KVM_EXIT_NMI
> exit reason upon a machine check exception (MCE) in
> the guest address space if the KVM_CAP_PPC_FWNMI
> capability is enabled (instead of delivering a 0x200
> interrupt to guest). This enables QEMU to build error
> log and deliver machine check exception to guest via
> guest registered machine check handler.
> 
> This approach simplifies the delivery of machine
> check exception to guest OS compared to the earlier
> approach of KVM directly invoking 0x200 guest interrupt
> vector.
> 
> This design/approach is based on the feedback for the
> QEMU patches to handle machine check exception. Details
> of earlier approach of handling machine check exception
> in QEMU and related discussions can be found at:
> 
> https://lists.nongnu.org/archive/html/qemu-devel/2014-11/msg00813.html
> 
> Note:
> 
> This patch introduces a hook which is invoked at the time
> of guest exit to facilitate the host-side handling of
> machine check exception before the exception is passed
> on to the guest. Hence, the host-side handling which was
> performed earlier via machine_check_fwnmi is removed.
> 
> The reasons for this approach is (i) it is not possible
> to distinguish whether the exception occurred in the
> guest or the host from the pt_regs passed on the
> machine_check_exception(). Hence machine_check_exception()
> calls panic, instead of passing on the exception to
> the guest, if the machine check exception is not
> recoverable. (ii) the approach introduced in this
> patch gives opportunity to the host kernel to perform
> actions in virtual mode before passing on the exception
> to the guest. This approach does not require complex
> tweaks to machine_check_fwnmi and friends.

It would be good to qualify the different types of MCE
and what action we expect across hypervisor and guest.

> 
> Signed-off-by: Aravinda Prasad <[email-masked]>
> ---
>  arch/powerpc/kvm/book3s_hv.c            |   27 +++++++++++++-----
>  arch/powerpc/kvm/book3s_hv_rmhandlers.S |   47 ++++++++++++++++---------------
>  arch/powerpc/platforms/powernv/opal.c   |   10 +++++++
>  3 files changed, 54 insertions(+), 30 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 3686471..cae4921 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -123,6 +123,7 @@ MODULE_PARM_DESC(halt_poll_ns_shrink, "Factor halt poll time is shrunk by");
>  
>  static void kvmppc_end_cede(struct kvm_vcpu *vcpu);
>  static int kvmppc_hv_setup_htab_rma(struct kvm_vcpu *vcpu);
> +static void kvmppc_machine_check_hook(void);
>  
>  static inline struct kvm_vcpu *next_runnable_thread(struct kvmppc_vcore *vc,
>  		int *ip)
> @@ -954,15 +955,14 @@ static int kvmppc_handle_exit_hv(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  		r = RESUME_GUEST;
>  		break;
>  	case BOOK3S_INTERRUPT_MACHINE_CHECK:
> +		/* Exit to guest with KVM_EXIT_NMI as exit reason */
> +		run->exit_reason = KVM_EXIT_NMI;
> +		r = RESUME_HOST;
>  		/*
> -		 * Deliver a machine check interrupt to the guest.
> -		 * We have to do this, even if the host has handled the
> -		 * machine check, because machine checks use SRR0/1 and
> -		 * the interrupt might have trashed guest state in them.
> +		 * Invoke host-kernel handler to perform any host-side
> +		 * handling before exiting the guest.
>  		 */
> -		kvmppc_book3s_queue_irqprio(vcpu,
> -					    BOOK3S_INTERRUPT_MACHINE_CHECK);
> -		r = RESUME_GUEST;
> +		kvmppc_machine_check_hook();
>  		break;
>  	case BOOK3S_INTERRUPT_PROGRAM:
>  	{
> @@ -3491,6 +3491,19 @@ static void kvmppc_irq_bypass_del_producer_hv(struct irq_bypass_consumer *cons,
>  }
>  #endif
>  
> +/*
> + * Hook to handle machine check exceptions occurred inside a guest.
> + * This hook is invoked from host virtual mode from KVM before exiting
> + * the guest with KVM_EXIT_NMI exit reason. This gives an opportunity
> + * for the host to take action (if any) before passing on the machine
> + * check exception to the guest kernel.
> + */
> +static void kvmppc_machine_check_hook(void)
> +{
> +	if (ppc_md.machine_check_exception)
> +		ppc_md.machine_check_exception(NULL);
> +}
> +
>  static long kvm_arch_vm_ioctl_hv(struct file *filp,
>  				 unsigned int ioctl, unsigned long arg)
>  {
> diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> index c3c1d1b..9b41390 100644
> --- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> +++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> @@ -134,21 +134,18 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
>  	stb	r0, HSTATE_HWTHREAD_REQ(r13)
>  
>  	/*
> -	 * For external and machine check interrupts, we need
> -	 * to call the Linux handler to process the interrupt.
> -	 * We do that by jumping to absolute address 0x500 for
> -	 * external interrupts, or the machine_check_fwnmi label
> -	 * for machine checks (since firmware might have patched
> -	 * the vector area at 0x200).  The [h]rfid at the end of the
> -	 * handler will return to the book3s_hv_interrupts.S code.
> -	 * For other interrupts we do the rfid to get back
> -	 * to the book3s_hv_interrupts.S code here.
> +	 * For external interrupts we need to call the Linux
> +	 * handler to process the interrupt. We do that by jumping
> +	 * to absolute address 0x500 for external interrupts.
> +	 * The [h]rfid at the end of the handler will return to
> +	 * the book3s_hv_interrupts.S code. For other interrupts
> +	 * we do the rfid to get back to the book3s_hv_interrupts.S
> +	 * code here.
>  	 */
>  	ld	r8, 112+PPC_LR_STKOFF(r1)
>  	addi	r1, r1, 112
>  	ld	r7, HSTATE_HOST_MSR(r13)
>  
> -	cmpwi	cr1, r12, BOOK3S_INTERRUPT_MACHINE_CHECK
>  	cmpwi	r12, BOOK3S_INTERRUPT_EXTERNAL
>  	beq	11f
>  	cmpwi	r12, BOOK3S_INTERRUPT_H_DOORBELL
> @@ -163,7 +160,10 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
>  	mtmsrd	r6, 1			/* Clear RI in MSR */
>  	mtsrr0	r8
>  	mtsrr1	r7
> -	beq	cr1, 13f		/* machine check */
> +	/*
> +	 * BOOK3S_INTERRUPT_MACHINE_CHECK is handled at the
> +	 * time of guest exit
> +	 */
>  	RFI
>  
>  	/* On POWER7, we have external interrupts set to use HSRR0/1 */
> @@ -171,8 +171,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
>  	mtspr	SPRN_HSRR1, r7
>  	ba	0x500
>  
> -13:	b	machine_check_fwnmi
> -
>  14:	mtspr	SPRN_HSRR0, r8
>  	mtspr	SPRN_HSRR1, r7
>  	b	hmi_exception_after_realmode
> @@ -2338,15 +2336,13 @@ machine_check_realmode:
>  	ld	r9, HSTATE_KVM_VCPU(r13)
>  	li	r12, BOOK3S_INTERRUPT_MACHINE_CHECK
>  	/*
> -	 * Deliver unhandled/fatal (e.g. UE) MCE errors to guest through
> -	 * machine check interrupt (set HSRR0 to 0x200). And for handled
> -	 * errors (no-fatal), just go back to guest execution with current
> -	 * HSRR0 instead of exiting guest. This new approach will inject
> -	 * machine check to guest for fatal error causing guest to crash.
> -	 *
> -	 * The old code used to return to host for unhandled errors which
> -	 * was causing guest to hang with soft lockups inside guest and
> -	 * makes it difficult to recover guest instance.
> +	 * Deliver unhandled/fatal (e.g. UE) MCE errors to guest either
> +	 * through machine check interrupt (set HSRR0 to 0x200) or by
> +	 * exiting the guest with KVM_EXIT_NMI exit reason if guest is
> +	 * FWNMI capable. For handled errors (no-fatal), just go back
> +	 * to guest execution with current HSRR0. This new approach
> +	 * injects machine check errors in guest address space to guest
> +	 * enabling guest kernel to suitably handle such errors.
>  	 *
>  	 * if we receive machine check with MSR(RI=0) then deliver it to
>  	 * guest as machine check causing guest to crash.
> @@ -2360,7 +2356,12 @@ machine_check_realmode:
>  	cmpdi	r3, 0		/* Did we handle MCE ? */
>  	bne	2f	/* Continue guest execution. */
>  	/* If not, deliver a machine check.  SRR0/1 are already set */
> -1:	li	r10, BOOK3S_INTERRUPT_MACHINE_CHECK
> +	/* Check if guest is capable of handling NMI exit */
> +1:	ld	r3, VCPU_KVM(r9)
> +	lbz	r3, KVM_FWNMI(r3)
> +	cmpdi	r3, 1       /* FWNMI capable? */
> +	beq	mc_cont
> +	li	r10, BOOK3S_INTERRUPT_MACHINE_CHECK
>  	bl	kvmppc_msr_interrupt
>  2:	b	fast_interrupt_c_return
>  
> diff --git a/arch/powerpc/platforms/powernv/opal.c b/arch/powerpc/platforms/powernv/opal.c
> index 6c9a65b..25749c6 100644
> --- a/arch/powerpc/platforms/powernv/opal.c
> +++ b/arch/powerpc/platforms/powernv/opal.c
> @@ -446,6 +446,16 @@ int opal_machine_check(struct pt_regs *regs)
>  	}
>  	machine_check_print_event_info(&evt);
>  
> +	/*
> +	 * If regs is NULL, then the machine check exception occurred
> +	 * in the guest. Currently no action is performed in the host
> +	 * other than printing the event information. The machine check
> +	 * exception is passed on to the guest kernel and the guest
> +	 * kernel will attempt for recovery.
> +	 */
> +	if (!regs)
> +		return 0;
> +

Shouldn't the host take action for example poison bad pages?

>  	if (opal_recover_mce(regs, &evt))
>  		return 1;
>  
>

Balbir Singh