Re: [PATCH v4 24/46] KVM: PPC: Book3S HV P9: Use large decrementer for HDEC

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On 23/03/2021 12:02, Nicholas Piggin wrote:
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@xxxxxxxxx>
Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>


ERROR: modpost: "decrementer_max" [arch/powerpc/kvm/kvm-hv.ko] undefined!


need this:

--- 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);


---
  arch/powerpc/include/asm/time.h | 2 ++
  arch/powerpc/kvm/book3s_hv.c    | 3 ++-
  2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/time.h b/arch/powerpc/include/asm/time.h
index 8dd3cdb25338..68d94711811e 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/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 8215430e6d5e..bb30c5ab53d1 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -3658,7 +3658,8 @@ static int kvmhv_load_hv_regs_and_go(struct kvm_vcpu *vcpu, u64 time_limit,
  		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);
switch_mmu_to_host_radix(kvm, host_pidr);

--
Alexey



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