Hi Gautam,

A few comments below ...

Gautam Menghani [off-list ref] writes:

Mask off the LPCR_MER bit before running a vCPU to ensure that it is not
set if there are no pending interrupts.

I would typically leave this until the end of the change log. ie.
describe the bug and how it happens first, then the fix at the end.

But it's not a hard rule, so up to you.

Running a vCPU with LPCR_MER bit

            ^
        "an L2 vCPU"

In general if you can qualify L0 vs L1 vs L2 everywhere it would help
folks follow the description.

set and no pending interrupts results in L2 vCPU getting an infinite flood
of spurious interrupts. The 'if check' in kvmhv_run_single_vcpu() sets
the LPCR_MER bit if there are pending interrupts.

The spurious flood problem can be observed in 2 cases:
1. Crashing the guest while interrupt heavy workload is running
  a. Start a L2 guest and run an interrupt heavy workload (eg: ipistorm)
  b. While the workload is running, crash the guest (make sure kdump
     is configured)
  c. Any one of the vCPUs of the guest will start getting an infinite
     flood of spurious interrupts.

2. Running LTP stress tests in multiple guests at the same time
   a. Start 4 L2 guests.
   b. Start running LTP stress tests on all 4 guests at same time.
   c. In some time, any one/more of the vCPUs of any of the guests will
      start getting an infinite flood of spurious interrupts.

The root cause of both the above issues is the same:
1. A NMI is sent to a running vCPU that has LPCR_MER bit set.
2. In the NMI path, all registers are refreshed, i.e, H_GUEST_GET_STATE
   is called for all the registers.
3. When H_GUEST_GET_STATE is called for lpcr, the vcpu->arch.vcore->lpcr
   of that vCPU at L1 level gets updated with LPCR_MER set to 1, and this
   new value is always used whenever that vCPU runs, regardless of whether
   there was a pending interrupt.
4. Since LPCR_MER is set, the vCPU in L2 always jumps to the external
   interrupt handler, and this cycle never ends.

Fix the spurious flood by making sure a vCPU's LPCR_MER is always masked
before running a vCPU.

I think your original sentence at the top of the change log is actually more
accurate. ie. it's not that LPCR_MER is always cleared, it's cleared
*unless there's a pending interrupt*.

Fixes: ec0f6639fa88 ("KVM: PPC: Book3S HV nestedv2: Ensure LPCR_MER bit is passed to the L0")
Cc: stable@vger.kernel.org # v6.8+
Signed-off-by: Gautam Menghani <redacted>
---
V1 -> V2:
1. Mask off the LPCR_MER in vcpu->arch.vcore->lpcr instead of resetting
it so that we avoid grabbing vcpu->arch.vcore->lock. (Suggested by
Ritesh in an internal review)

Did v1 take the vcore->lock? I don't remember it.

                                     ^
                                     LPCR

+		 * unexpectedly set - for e.g. during NMI handling when all register
+		 * states are synchronized from L0 to L1. L1 needs to inform L0 about
+		 * MER=1 only when there are pending external interrupts.
+		 * kvmhv_run_single_vcpu() anyway sets MER bit if there are pending
+		 * external interrupts. Hence, mask off MER bit when passing vcore->lpcr
+		 * here as otherwise it may generate spurious interrupts in L2 KVM
+		 * causing an endless loop, which results in L2 guest getting hung.
+		 */
 		if (cpu_has_feature(CPU_FTR_ARCH_300))
 			r = kvmhv_run_single_vcpu(vcpu, ~(u64)0,
-						  vcpu->arch.vcore->lpcr);
+						  vcpu->arch.vcore->lpcr & ~LPCR_MER);

 
This is much better than v1 which hid the clearing of LPCR_MER in a macro.

But I still wonder if it would be better to clear it in
kvmhv_run_single_vcpu() itself.

The logic to set LPCR_MER is already in there, so why not ensure
LPCR_MER is cleared as part of that some block?

I realise there's another caller of kvmhv_run_single_vcpu() from the
nested code, but that's OK because there's already a nested check in
kvmhv_run_single_vcpu(), so you can still isolate this change to just
the non-nested case.

cheers