Re: [RFC PATCH 38/43] KVM: PPC: Book3S HV P9: Test dawr_enabled() before saving host DAWR SPRs

Excerpts from Fabiano Rosas's message of July 1, 2021 3:51 am:

Nicholas Piggin [off-list ref] writes:

quoted

Some of the DAWR SPR access is already predicated on dawr_enabled(),
apply this to the remainder of the accesses.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/kvm/book3s_hv_p9_entry.c | 34 ++++++++++++++++-----------
 1 file changed, 20 insertions(+), 14 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv_p9_entry.c b/arch/powerpc/kvm/book3s_hv_p9_entry.c
index 7aa72efcac6c..f305d1d6445c 100644
--- a/arch/powerpc/kvm/book3s_hv_p9_entry.c
+++ b/arch/powerpc/kvm/book3s_hv_p9_entry.c

@@ -638,13 +638,16 @@ int kvmhv_vcpu_entry_p9(struct kvm_vcpu *vcpu, u64 time_limit, unsigned long lpc

 	host_hfscr = mfspr(SPRN_HFSCR);
 	host_ciabr = mfspr(SPRN_CIABR);
-	host_dawr0 = mfspr(SPRN_DAWR0);
-	host_dawrx0 = mfspr(SPRN_DAWRX0);
 	host_psscr = mfspr(SPRN_PSSCR);
 	host_pidr = mfspr(SPRN_PID);
-	if (cpu_has_feature(CPU_FTR_DAWR1)) {
-		host_dawr1 = mfspr(SPRN_DAWR1);
-		host_dawrx1 = mfspr(SPRN_DAWRX1);
+
+	if (dawr_enabled()) {
+		host_dawr0 = mfspr(SPRN_DAWR0);
+		host_dawrx0 = mfspr(SPRN_DAWRX0);
+		if (cpu_has_feature(CPU_FTR_DAWR1)) {
+			host_dawr1 = mfspr(SPRN_DAWR1);
+			host_dawrx1 = mfspr(SPRN_DAWRX1);

The userspace needs to enable DAWR1 via KVM_CAP_PPC_DAWR1. That cap is
not even implemented in QEMU currently, so we never allow the guest to
set vcpu->arch.dawr1. If we check for kvm->arch.dawr1_enabled instead of
the CPU feature, we could shave some more time here.

Ah good point, yes let's do that.

Thanks,
Nick