On 01/16/2017 10:05 AM, Paul Mackerras wrote:

On Fri, Jan 13, 2017 at 04:51:45PM +0530, Aravinda Prasad wrote:

quoted

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.

Signed-off-by: Aravinda Prasad <redacted>
Reviewed-by: David Gibson <redacted>

This patch mostly looks OK.  I have a few relatively minor comments
below.

quoted

---
 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();

Note that this won't necessarily be called on the same CPU that
received the machine check.  This will be called on thread 0 of the
core (or subcore), whereas the machine check could have occurred on
some other thread.  Are you sure that the machine check handling code
will be OK with that?

That will have only one problem. get_mce_event() from
opal_machine_check() may not be able to pull mce event for error on
non-zero thread. We should hook the mce event into vcpu structure during
kvmppc_realmode_machine_check() and then pass it to
ppc_md.machine_check_exception() as an additional argument.

quoted

 		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);
+}

What is the advantage of having this as a separate function, as
opposed to just putting those two lines of code in line in the one
place where this gets called?

Paul.