[PATCH stable v4.14 23/32] powerpc/fsl: Emulate SPRN_BUCSR register

Michael Ellerman mpe at ellerman.id.au
Fri Mar 29 22:26:11 AEDT 2019

From: Diana Craciun <diana.craciun at nxp.com>

commit 98518c4d8728656db349f875fcbbc7c126d4c973 upstream.

In order to flush the branch predictor the guest kernel performs
writes to the BUCSR register which is hypervisor privilleged. However,
the branch predictor is flushed at each KVM entry, so the branch
predictor has been already flushed, so just return as soon as possible
to guest.

Signed-off-by: Diana Craciun <diana.craciun at nxp.com>
[mpe: Tweak comment formatting]
Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
 arch/powerpc/kvm/e500_emulate.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/powerpc/kvm/e500_emulate.c b/arch/powerpc/kvm/e500_emulate.c
index 990db69a1d0b..fa88f641ac03 100644
--- a/arch/powerpc/kvm/e500_emulate.c
+++ b/arch/powerpc/kvm/e500_emulate.c
@@ -277,6 +277,13 @@ int kvmppc_core_emulate_mtspr_e500(struct kvm_vcpu *vcpu, int sprn, ulong spr_va
 		vcpu->arch.pwrmgtcr0 = spr_val;
+	case SPRN_BUCSR:
+		/*
+		 * If we are here, it means that we have already flushed the
+		 * branch predictor, so just return to guest.
+		 */
+		break;
 	/* extra exceptions */
 	case SPRN_IVOR32:

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