[PATCH] powerpc: kvm: optimize "sc 0" as fast return

[Liu Ping Fan]

syscall is a very common behavior inside guest, and this patch
optimizes the path for the emulation of BOOK3S_INTERRUPT_SYSCALL,
so hypervisor can return to guest without heavy exit, i.e, no need
to swap TLB, HTAB,.. etc

Signed-off-by: Liu Ping Fan <pingfank at linux.vnet.ibm.com>
---
Compiled, but lack of bare metal, I have not tested it yet.
---
 arch/powerpc/kvm/book3s_hv.c            |  6 ------
 arch/powerpc/kvm/book3s_hv_rmhandlers.S | 13 ++++++++++++-
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 62a2b5a..73dc852 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -628,12 +628,6 @@ static int kvmppc_handle_exit(struct kvm_run *run, struct kvm_vcpu *vcpu,
 		/* hcall - punt to userspace */
 		int i;
 
-		if (vcpu->arch.shregs.msr & MSR_PR) {
-			/* sc 1 from userspace - reflect to guest syscall */
-			kvmppc_book3s_queue_irqprio(vcpu, BOOK3S_INTERRUPT_SYSCALL);
-			r = RESUME_GUEST;
-			break;
-		}
 		run->papr_hcall.nr = kvmppc_get_gpr(vcpu, 3);
 		for (i = 0; i < 9; ++i)
 			run->papr_hcall.args[i] = kvmppc_get_gpr(vcpu, 4 + i);
diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index c71103b..9f626c3 100644
--- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
+++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
@@ -1388,7 +1388,8 @@ kvmppc_hisi:
 hcall_try_real_mode:
 	ld	r3,VCPU_GPR(R3)(r9)
 	andi.	r0,r11,MSR_PR
-	bne	guest_exit_cont
+	/* sc 1 from userspace - reflect to guest syscall */
+	bne	sc_0_fast_return
 	clrrdi	r3,r3,2
 	cmpldi	r3,hcall_real_table_end - hcall_real_table
 	bge	guest_exit_cont
@@ -1409,6 +1410,16 @@ hcall_try_real_mode:
 	ld	r11,VCPU_MSR(r4)
 	b	fast_guest_return
 
+sc_0_fast_return:
+	ld	r10,VCPU_PC(r9)
+	ld	r11,VCPU_MSR(r9)
+	mtspr	SPRN_SRR0,r10
+	mtspr	SPRN_SRR1,r11
+	li	r10, BOOK3S_INTERRUPT_SYSCALL
+	LOAD_REG_IMMEDIATE(r3,0xffffffff87a0ffff)	/* zero 33:36,42:47 */
+	and	r11,r11,r3
+	b	fast_guest_return
+
 	/* We've attempted a real mode hcall, but it's punted it back
 	 * to userspace.  We need to restore some clobbered volatiles
 	 * before resuming the pass-it-to-qemu path */
-- 
1.8.1.4