[PATCH 27/37] KVM: arm64: Defer saving/restoring system registers to vcpu load/put on VHE

[Christoffer Dall]

Some system registers do not affect the host kernel's execution and can
therefore be loaded when we are about to run a VCPU and we don't have to
restore the host state to the hardware before the time when we are
actually about to return to userspace or schedule out the VCPU thread.

The EL1 system registers and the userspace state registers, which only
affect EL0 execution, do not affect the host kernel's execution.

The 32-bit system registers are not used by a VHE host kernel and
therefore don't need to be saved/restored on every entry/exit to/from
the guest, but can be deferred to vcpu_load and vcpu_put, respectively.

We have already prepared the trap handling code which accesses any of
these registers to directly access the registers on the physical CPU or
to sync the registers when needed.

Signed-off-by: Christoffer Dall <christoffer.dall at linaro.org>
---
 arch/arm64/kvm/hyp/switch.c    |  6 ------
 arch/arm64/kvm/hyp/sysreg-sr.c | 46 ++++++++++++++++++++++++++++++++++--------
 2 files changed, 38 insertions(+), 14 deletions(-)

diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
index 6356bec..6a12504 100644
--- a/arch/arm64/kvm/hyp/switch.c
+++ b/arch/arm64/kvm/hyp/switch.c
@@ -337,11 +337,6 @@ int kvm_vcpu_run(struct kvm_vcpu *vcpu)
 
 	__vgic_restore_state(vcpu);
 
-	/*
-	 * We must restore the 32-bit state before the sysregs, thanks
-	 * to erratum #852523 (Cortex-A57) or #853709 (Cortex-A72).
-	 */
-	__sysreg32_restore_state(vcpu);
 	sysreg_restore_guest_state_vhe(guest_ctxt);
 	__debug_switch_to_guest(vcpu);
 
@@ -354,7 +349,6 @@ int kvm_vcpu_run(struct kvm_vcpu *vcpu)
 		goto again;
 
 	sysreg_save_guest_state_vhe(guest_ctxt);
-	__sysreg32_save_state(vcpu);
 	__vgic_save_state(vcpu);
 
 	__deactivate_traps(vcpu);
diff --git a/arch/arm64/kvm/hyp/sysreg-sr.c b/arch/arm64/kvm/hyp/sysreg-sr.c
index 354ca02..6ff1ce5 100644
--- a/arch/arm64/kvm/hyp/sysreg-sr.c
+++ b/arch/arm64/kvm/hyp/sysreg-sr.c
@@ -25,8 +25,12 @@
 /*
  * Non-VHE: Both host and guest must save everything.
  *
- * VHE: Host must save tpidr*_el0, actlr_el1, mdscr_el1, sp_el0,
- * and guest must save everything.
+ * VHE: Host and guest must save mdscr_el1 and sp_el0 (and the PC and pstate,
+ * which are handled as part of the el2 return state) on every switch.
+ * tpidr_el0, tpidrro_el0, and actlr_el1 only need to be switched when going
+ * to host userspace or a different VCPU.  EL1 registers only need to be
+ * switched when potentially going to run a different VCPU.  The latter two
+ * classes are handled as part of kvm_arch_vcpu_load and kvm_arch_vcpu_put.
  */
 
 static void __hyp_text __sysreg_save_common_state(struct kvm_cpu_context *ctxt)
@@ -85,14 +89,11 @@ void __hyp_text __sysreg_save_state_nvhe(struct kvm_cpu_context *ctxt)
 void sysreg_save_host_state_vhe(struct kvm_cpu_context *ctxt)
 {
 	__sysreg_save_common_state(ctxt);
-	__sysreg_save_user_state(ctxt);
 }
 
 void sysreg_save_guest_state_vhe(struct kvm_cpu_context *ctxt)
 {
-	__sysreg_save_el1_state(ctxt);
 	__sysreg_save_common_state(ctxt);
-	__sysreg_save_user_state(ctxt);
 	__sysreg_save_el2_return_state(ctxt);
 }
 
@@ -153,14 +154,11 @@ void __hyp_text __sysreg_restore_state_nvhe(struct kvm_cpu_context *ctxt)
 void sysreg_restore_host_state_vhe(struct kvm_cpu_context *ctxt)
 {
 	__sysreg_restore_common_state(ctxt);
-	__sysreg_restore_user_state(ctxt);
 }
 
 void sysreg_restore_guest_state_vhe(struct kvm_cpu_context *ctxt)
 {
-	__sysreg_restore_el1_state(ctxt);
 	__sysreg_restore_common_state(ctxt);
-	__sysreg_restore_user_state(ctxt);
 	__sysreg_restore_el2_return_state(ctxt);
 }
 
@@ -224,6 +222,26 @@ void __hyp_text __sysreg32_restore_state(struct kvm_vcpu *vcpu)
  */
 void kvm_vcpu_load_sysregs(struct kvm_vcpu *vcpu)
 {
+	struct kvm_cpu_context *host_ctxt = vcpu->arch.host_cpu_context;
+	struct kvm_cpu_context *guest_ctxt = &vcpu->arch.ctxt;
+
+	if (!has_vhe())
+		return;
+
+	__sysreg_save_user_state(host_ctxt);
+
+
+	/*
+	 * Load guest EL1 and user state
+	 *
+	 * We must restore the 32-bit state before the sysregs, thanks
+	 * to erratum #852523 (Cortex-A57) or #853709 (Cortex-A72).
+	 */
+	__sysreg32_restore_state(vcpu);
+	__sysreg_restore_user_state(guest_ctxt);
+	__sysreg_restore_el1_state(guest_ctxt);
+
+	vcpu->arch.sysregs_loaded_on_cpu = true;
 }
 
 /**
@@ -250,4 +268,16 @@ void kvm_vcpu_put_sysregs(struct kvm_vcpu *vcpu)
 		__fpsimd_restore_state(&host_ctxt->gp_regs.fp_regs);
 		vcpu->arch.guest_vfp_loaded = 0;
 	}
+
+	if (!has_vhe())
+		return;
+
+	__sysreg_save_el1_state(guest_ctxt);
+	__sysreg_save_user_state(guest_ctxt);
+	__sysreg32_save_state(vcpu);
+
+	/* Restore host user state */
+	__sysreg_restore_user_state(host_ctxt);
+
+	vcpu->arch.sysregs_loaded_on_cpu = false;
 }
-- 
2.9.0