KVM: PPC: Book3S HV: Perserve PSSCR FAKE_SUSPEND bit on guest exit

There is a hardware bug in some POWER9 processors where a treclaim in
fake suspend mode can cause an inconsistency in the XER[SO] bit across
the threads of a core, the workaround being to force the core into SMT4
when doing the treclaim.

The FAKE_SUSPEND bit (bit 10) in the PSSCR is used to control whether a
thread is in fake suspend or real suspend. The important difference here
being that thread reconfiguration is blocked in real suspend but not
fake suspend mode.

When we exit a guest which was in fake suspend mode, we force the core
into SMT4 while we do the treclaim in kvmppc_save_tm_hv().
However on the new exit path introduced with the function
kvmhv_run_single_vcpu() we restore the host PSSCR before calling
kvmppc_save_tm_hv() which means that if we were in fake suspend mode we
put the thread into real suspend mode when we clear the
PSSCR[FAKE_SUSPEND] bit. This means that we block thread reconfiguration
and the thread which is trying to get the core into SMT4 before it can
do the treclaim spins forever since it itself is blocking thread
reconfiguration. The result is that that core is essentially lost.

This results in a trace such as:
[   93.512904] CPU: 7 PID: 13352 Comm: qemu-system-ppc Not tainted 5.0.0 #4
[   93.512905] NIP:  c000000000098a04 LR: c0000000000cc59c CTR: 0000000000000000
[   93.512908] REGS: c000003fffd2bd70 TRAP: 0100   Not tainted  (5.0.0)
[   93.512908] MSR:  9000000302883033 <SF,HV,VEC,VSX,FP,ME,IR,DR,RI,LE,TM[SE]>  CR: 22222444  XER: 00000000
[   93.512914] CFAR: c000000000098a5c IRQMASK: 3
[   93.512915] PACATMSCRATCH: 0000000000000001
[   93.512916] GPR00: 0000000000000001 c000003f6cc1b830 c000000001033100 0000000000000004
[   93.512928] GPR04: 0000000000000004 0000000000000002 0000000000000004 0000000000000007
[   93.512930] GPR08: 0000000000000000 0000000000000004 0000000000000000 0000000000000004
[   93.512932] GPR12: c000203fff7fc000 c000003fffff9500 0000000000000000 0000000000000000
[   93.512935] GPR16: 2000000000300375 000000000000059f 0000000000000000 0000000000000000
[   93.512951] GPR20: 0000000000000000 0000000000080053 004000000256f41f c000003f6aa88ef0
[   93.512953] GPR24: c000003f6aa89100 0000000000000010 0000000000000000 0000000000000000
[   93.512956] GPR28: c000003f9e9a0800 0000000000000000 0000000000000001 c000203fff7fc000
[   93.512959] NIP [c000000000098a04] pnv_power9_force_smt4_catch+0x1b4/0x2c0
[   93.512960] LR [c0000000000cc59c] kvmppc_save_tm_hv+0x40/0x88
[   93.512960] Call Trace:
[   93.512961] [c000003f6cc1b830] [0000000000080053] 0x80053 (unreliable)
[   93.512965] [c000003f6cc1b8a0] [c00800001e9cb030] kvmhv_p9_guest_entry+0x508/0x6b0 [kvm_hv]
[   93.512967] [c000003f6cc1b940] [c00800001e9cba44] kvmhv_run_single_vcpu+0x2dc/0xb90 [kvm_hv]
[   93.512968] [c000003f6cc1ba10] [c00800001e9cc948] kvmppc_vcpu_run_hv+0x650/0xb90 [kvm_hv]
[   93.512969] [c000003f6cc1bae0] [c00800001e8f620c] kvmppc_vcpu_run+0x34/0x48 [kvm]
[   93.512971] [c000003f6cc1bb00] [c00800001e8f2d4c] kvm_arch_vcpu_ioctl_run+0x2f4/0x400 [kvm]
[   93.512972] [c000003f6cc1bb90] [c00800001e8e3918] kvm_vcpu_ioctl+0x460/0x7d0 [kvm]
[   93.512974] [c000003f6cc1bd00] [c0000000003ae2c0] do_vfs_ioctl+0xe0/0x8e0
[   93.512975] [c000003f6cc1bdb0] [c0000000003aeb24] ksys_ioctl+0x64/0xe0
[   93.512978] [c000003f6cc1be00] [c0000000003aebc8] sys_ioctl+0x28/0x80
[   93.512981] [c000003f6cc1be20] [c00000000000b3a4] system_call+0x5c/0x70
[   93.512983] Instruction dump:
[   93.512986] 419dffbc e98c0000 2e8b0000 38000001 60000000 60000000 60000000 40950068
[   93.512993] 392bffff 39400000 79290020 39290001 <7d2903a6> 60000000 60000000 7d235214

To fix this we preserve the PSSCR[FAKE_SUSPEND] bit until we call
kvmppc_save_tm_hv() which will mean the core can get into SMT4 and
perform the treclaim. Note kvmppc_save_tm_hv() clears the
PSSCR[FAKE_SUSPEND] bit again so there is no need to explicitly do that.

Fixes: 95a6432c ("KVM: PPC: Book3S HV: Streamlined guest entry/exit path on P9 for radix guests")
Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

arch/powerpc/kvm/book3s_hv.c

@@ -3423,7 +3423,9 @@ static int kvmhv_load_hv_regs_and_go(struct kvm_vcpu *vcpu, u64 time_limit,
 	vcpu->arch.shregs.sprg2 = mfspr(SPRN_SPRG2);
 	vcpu->arch.shregs.sprg3 = mfspr(SPRN_SPRG3);
 
-	mtspr(SPRN_PSSCR, host_psscr);
+	/* Preserve PSSCR[FAKE_SUSPEND] until we've called kvmppc_save_tm_hv */
+	mtspr(SPRN_PSSCR, host_psscr |
+	      (local_paca->kvm_hstate.fake_suspend << PSSCR_FAKE_SUSPEND_LG));
 	mtspr(SPRN_HFSCR, host_hfscr);
 	mtspr(SPRN_CIABR, host_ciabr);
 	mtspr(SPRN_DAWR, host_dawr);