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

[Paul Mackerras <paulus@ozlabs.org>]

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.

Paul.