[PATCH] powerpc/kvm: Fix kvmppc_vcore->in_guest value in kvmhv_switch_to_host

[Jordan Niethe <jniethe5@gmail.com>]

kvmhv_switch_to_host() in arch/powerpc/kvm/book3s_hv_rmhandlers.S needs
to set kvmppc_vcore->in_guest to 0 to signal secondary CPUs to continue.
This happens after resetting the PCR. Before commit 13c7bb3c57dc
("powerpc/64s: Set reserved PCR bits"), r0 would always be 0 before it
was stored to kvmppc_vcore->in_guest. However because of this change in
the commit:

        /* Reset PCR */
        ld      r0, VCORE_PCR(r5)
-       cmpdi   r0, 0
+       LOAD_REG_IMMEDIATE(r6, PCR_MASK)
+       cmpld   r0, r6
        beq     18f
-       li      r0, 0
-       mtspr   SPRN_PCR, r0
+       mtspr   SPRN_PCR, r6
 18:
        /* Signal secondary CPUs to continue */
        stb     r0,VCORE_IN_GUEST(r5)

We are no longer comparing r0 against 0 and loading it with 0 if it
contains something else. Hence when we store r0 to
kvmppc_vcore->in_guest, it might not be 0.  This means that secondary
CPUs will not be signalled to continue. Those CPUs get stuck and errors
like the following are logged:

    KVM: CPU 1 seems to be stuck
    KVM: CPU 2 seems to be stuck
    KVM: CPU 3 seems to be stuck
    KVM: CPU 4 seems to be stuck
    KVM: CPU 5 seems to be stuck
    KVM: CPU 6 seems to be stuck
    KVM: CPU 7 seems to be stuck

This can be reproduced with:
    $ for i in `seq 1 7` ; do chcpu -d $i ; done ;
    $ taskset -c 0 qemu-system-ppc64 -smp 8,threads=8 \
       -M pseries,accel=kvm,kvm-type=HV -m 1G -nographic -vga none \
       -kernel vmlinux -initrd initrd.cpio.xz

Fix by making sure r0 is 0 before storing it to kvmppc_vcore->in_guest.

Fixes: 13c7bb3c57dc ("powerpc/64s: Set reserved PCR bits")
Reported-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
---
 arch/powerpc/kvm/book3s_hv_rmhandlers.S | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index 74a9cfe84aee..faebcbb8c4db 100644
--- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
+++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
@@ -1921,6 +1921,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
 	mtspr	SPRN_PCR, r6
 18:
 	/* Signal secondary CPUs to continue */
+	li	r0, 0
 	stb	r0,VCORE_IN_GUEST(r5)
 19:	lis	r8,0x7fff		/* MAX_INT@h */
 	mtspr	SPRN_HDEC,r8
-- 
2.20.1