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

On Sunday 24 January 2016 02:54 AM, Paul Mackerras wrote:

On Sat, Jan 23, 2016 at 06:23:35PM +0530, Aravinda Prasad wrote:

quoted

On Saturday 23 January 2016 03:58 PM, Paul Mackerras wrote:

quoted

On Wed, Jan 13, 2016 at 12:38:09PM +0530, Aravinda Prasad wrote:

quoted

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]

quoted

--- 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.
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-- 
Regards,
Aravinda