[RFC PATCH 04/43] KVM: PPC: Book3S HV P9: Use large decrementer for HDEC

[Nicholas Piggin]

On processors that don't suppress the HDEC exceptions when LPCR[HDICE]=0,
this could help reduce needless guest exits due to leftover exceptions on
entering the guest.

Reviewed-by: Alexey Kardashevskiy <aik at ozlabs.ru>
Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
---
 arch/powerpc/include/asm/time.h       | 2 ++
 arch/powerpc/kernel/time.c            | 1 +
 arch/powerpc/kvm/book3s_hv_p9_entry.c | 3 ++-
 3 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/time.h b/arch/powerpc/include/asm/time.h
index fd09b4797fd7..69b6be617772 100644
--- a/arch/powerpc/include/asm/time.h
+++ b/arch/powerpc/include/asm/time.h
@@ -18,6 +18,8 @@
 #include <asm/vdso/timebase.h>
 
 /* time.c */
+extern u64 decrementer_max;
+
 extern unsigned long tb_ticks_per_jiffy;
 extern unsigned long tb_ticks_per_usec;
 extern unsigned long tb_ticks_per_sec;
diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 98bdd96141f2..026b3c0b648c 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -89,6 +89,7 @@ static struct clocksource clocksource_timebase = {
 
 #define DECREMENTER_DEFAULT_MAX 0x7FFFFFFF
 u64 decrementer_max = DECREMENTER_DEFAULT_MAX;
+EXPORT_SYMBOL_GPL(decrementer_max); /* for KVM HDEC */
 
 static int decrementer_set_next_event(unsigned long evt,
 				      struct clock_event_device *dev);
diff --git a/arch/powerpc/kvm/book3s_hv_p9_entry.c b/arch/powerpc/kvm/book3s_hv_p9_entry.c
index 83f592eadcd2..63afd277c5f3 100644
--- a/arch/powerpc/kvm/book3s_hv_p9_entry.c
+++ b/arch/powerpc/kvm/book3s_hv_p9_entry.c
@@ -489,7 +489,8 @@ int kvmhv_vcpu_entry_p9(struct kvm_vcpu *vcpu, u64 time_limit, unsigned long lpc
 		vc->tb_offset_applied = 0;
 	}
 
-	mtspr(SPRN_HDEC, 0x7fffffff);
+	/* HDEC must be at least as large as DEC, so decrementer_max fits */
+	mtspr(SPRN_HDEC, decrementer_max);
 
 	save_clear_guest_mmu(kvm, vcpu);
 	switch_mmu_to_host(kvm, host_pidr);
-- 
2.23.0