powerpc/pseries: Add KVM guest doorbell restrictions

KVM guests have certain restrictions and performance quirks when using
doorbells. This patch moves the EPAPR KVM guest test so it can be shared
with PSERIES, and uses that in doorbell setup code to apply the KVM
guest quirks and  improves IPI performance for two cases:

 - PowerVM guests may now use doorbells even if they are secure.

 - KVM guests no longer use doorbells if XIVE is available.

There is a valid complaint that "KVM guest" is not a very reasonable
thing to test for, it's preferable for the hypervisor to advertise
particular behaviours to the guest so they could change if the
hypervisor implementation or configuration changes. However in this case
we were already assuming a KVM guest worst case, so this patch is about
containing those quirks. If KVM later advertises fast doorbells, we
should test for that and override the quirks.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Tested-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200726035155.1424103-4-npiggin@gmail.com

1 files changed, 23 insertions, 15 deletions

diff --git a/arch/powerpc/platforms/pseries/smp.c b/arch/powerpc/platforms/pseries/smp.c
index a1cb861154e9..92922491a81c 100644
--- a/arch/powerpc/platforms/pseries/smp.c
+++ b/arch/powerpc/platforms/pseries/smp.c
@@ -210,24 +210,32 @@ static __init void pSeries_smp_probe(void)
 	if (!cpu_has_feature(CPU_FTR_SMT))
 		return;
 
-	/*
-	 * KVM emulates doorbells by disabling FSCR[MSGP] so msgsndp faults
-	 * to the hypervisor which then reads the instruction from guest
-	 * memory. This can't be done if the guest is secure, so don't use
-	 * doorbells in secure guests.
-	 *
-	 * Under PowerVM, FSCR[MSGP] is enabled so doorbells could be used
-	 * by secure guests if we distinguished this from KVM.
-	 */
-	if (is_secure_guest())
-		return;
+	if (is_kvm_guest()) {
+		/*
+		 * KVM emulates doorbells by disabling FSCR[MSGP] so msgsndp
+		 * faults to the hypervisor which then reads the instruction
+		 * from guest memory, which tends to be slower than using XIVE.
+		 */
+		if (xive_enabled())
+			return;
+
+		/*
+		 * XICS hcalls aren't as fast, so we can use msgsndp (which
+		 * also helps exercise KVM emulation), however KVM can't
+		 * emulate secure guests because it can't read the instruction
+		 * out of their memory.
+		 */
+		if (is_secure_guest())
+			return;
+	}
 
 	/*
-	 * The guest can use doobells for SMT sibling IPIs, which stay in
-	 * the core rather than going to the interrupt controller. This
-	 * tends to be slower under KVM where doorbells are emulated, but
-	 * faster for PowerVM where they're enabled.
+	 * Under PowerVM, FSCR[MSGP] is enabled as guest vCPU siblings are
+	 * gang scheduled on the same physical core, so doorbells are always
+	 * faster than the interrupt controller, and they can be used by
+	 * secure guests.
 	 */
+
 	ic_cause_ipi = smp_ops->cause_ipi;
 	smp_ops->cause_ipi = dbell_or_ic_cause_ipi;
 }