Subject: [PATCH v5 23/48] KVM: PPC: Book3S HV P9: Move setting HDEC after switching to guest LPCR

From: Nicholas Piggin <[email-masked]>
Date: Fri, 2 Apr 2021 01:03:00 +1000

LPCR[HDICE]=0 suppresses hypervisor decrementer exceptions on some
processors, so it must be enabled before HDEC is set.

Rather than set it in the host LPCR then setting HDEC, move the HDEC
update to after the guest MMU context (including LPCR) is loaded.
There shouldn't be much concern with delaying HDEC by some 10s or 100s
of nanoseconds by setting it a bit later.

Reviewed-by: Alexey Kardashevskiy <[email-masked]>
Reviewed-by: Fabiano Rosas <[email-masked]>
Signed-off-by: Nicholas Piggin <[email-masked]>
---
 arch/powerpc/kvm/book3s_hv.c | 19 +++++++------------
 1 file changed, 7 insertions(+), 12 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index c45352fbc25d..c249f77ea08c 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -3551,20 +3551,9 @@ static int kvmhv_load_hv_regs_and_go(struct kvm_vcpu *vcpu, u64 time_limit,
 		host_dawrx1 = mfspr(SPRN_DAWRX1);
 	}
 
-	/*
-	 * P8 and P9 suppress the HDEC exception when LPCR[HDICE] = 0,
-	 * so set HDICE before writing HDEC.
-	 */
-	mtspr(SPRN_LPCR, kvm->arch.host_lpcr | LPCR_HDICE);
-	isync();
-
 	hdec = time_limit - mftb();
-	if (hdec < 0) {
-		mtspr(SPRN_LPCR, kvm->arch.host_lpcr);
-		isync();
+	if (hdec < 0)
 		return BOOK3S_INTERRUPT_HV_DECREMENTER;
-	}
-	mtspr(SPRN_HDEC, hdec);
 
 	if (vc->tb_offset) {
 		u64 new_tb = mftb() + vc->tb_offset;
@@ -3610,6 +3599,12 @@ static int kvmhv_load_hv_regs_and_go(struct kvm_vcpu *vcpu, u64 time_limit,
 
 	switch_mmu_to_guest_radix(kvm, vcpu, lpcr);
 
+	/*
+	 * P9 suppresses the HDEC exception when LPCR[HDICE] = 0,
+	 * so set guest LPCR (with HDICE) before writing HDEC.
+	 */
+	mtspr(SPRN_HDEC, hdec);
+
 	mtspr(SPRN_SRR0, vcpu->arch.shregs.srr0);
 	mtspr(SPRN_SRR1, vcpu->arch.shregs.srr1);
 
-- 
2.23.0