Subject: Re: [PATCH v3 2/2] KVM: PPC: Exit guest upon MCE when FWNMI capability is enabled

From: Aravinda Prasad <[email-masked]>
Date: Mon, 25 Jan 2016 14:09:30 +0530

On Sunday 24 January 2016 02:54 AM, Paul Mackerras wrote:
> On Sat, Jan 23, 2016 at 06:23:35PM +0530, Aravinda Prasad wrote:
>>
>>
>> On Saturday 23 January 2016 03:58 PM, Paul Mackerras wrote:
>>> On Wed, Jan 13, 2016 at 12:38:09PM +0530, Aravinda Prasad wrote:
>>>> Enhance KVM to cause a guest exit with KVM_EXIT_NMI
>>>> exit reasons upon a machine check exception (MCE) in
>>>> the guest address space if the KVM_CAP_PPC_FWNMI
>>>> capability is enabled (instead of delivering 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 delivering of machine
>>>> check exception to guest OS compared to the earlier
>>>> approach of KVM directly invoking 0x200 guest interrupt
>>>> vector. In the earlier approach QEMU was enhanced to
>>>> patch the 0x200 interrupt vector during boot. The
>>>> patched code at 0x200 issued a private hcall to pass
>>>> the control to QEMU to build the error log.
>>>>
>>>> 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:
>>>
>>> [snip]
>>>
>>>> --- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
>>>> +++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
>>>> @@ -133,21 +133,18 @@ 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
>>>> @@ -162,7 +159,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
>>>>  	mtmsrd	r6, 1			/* Clear RI in MSR */
>>>>  	mtsrr0	r8
>>>>  	mtsrr1	r7
>>>> -	beq	cr1, 13f		/* machine check */
>>>>  	RFI
>>>>  
>>>>  	/* On POWER7, we have external interrupts set to use HSRR0/1 */
>>>> @@ -170,8 +166,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
>>>>  	mtspr	SPRN_HSRR1, r7
>>>>  	ba	0x500
>>>>  
>>>> -13:	b	machine_check_fwnmi
>>>> -
>>>
>>> So, what you're disabling here is the host-side handling of the
>>> machine check after completing the guest->host switch.  This has
>>> nothing to do with how the machine check gets communicated to the
>>> guest.
>>>
>>> Now, part of the host-side machine check handling has already
>>> happened, but I thought there was more that was done in host kernel
>>> virtual mode.  If this change really is needed then I would want an
>>> ack from Mahesh that this is correct, and it will need to be explained
>>> in detail in the patch description.
>>
>> If we don't do that we will end up running into
>> panic() in opal_machine_check() if UE belonged to guest.
>>
>> Details in this link:
>> http://marc.info/?l=kvm-ppc&m=144730552720044&w=2
> 
> Well maybe the panic call needs to be changed.  But the way you have
> it, we *never* get to opal_machine_check for any machine check
> interrupt, and I have a hard time believing that is correct.

If I am not wrong, earlier, even without this patch we never get to
opal_machine_check() from this place for the machine check happening
inside the guest (irrespective of whether the machine check is recovered
or not). Because, we jump to fast_interrupt_c_return() from
machine_check_realmode without causing the guest to exit.

With this patch I call mc_cont from machine_check_realmode and want the
guest to exit with NMI exit code. However, we don't want to call
opal_machine_check(), because if we call opal_machine_check() we don't
end up with NMI exit for the guest.

Regards,
Aravinda

> 
> Paul.
> _______________________________________________
> Linuxppc-dev mailing list
> [email-masked]
> https://lists.ozlabs.org/listinfo/linuxppc-dev
> 

-- 
Regards,
Aravinda