[PATCH 06/10] KVM: arm64: Avoid needlessly reloading guest FP state

[Andrew Scull <ascull@google.com>]

When returning to a user task from a vcpu, keep track of the vcpu state
being in registers so that the state can be reinstated on the next entry
if the registers still contain the vcpu's latest state.

This avoids the need to trap and restore in the case that the vcpu's
registers are still in place. The state must still be saved when
switching away from the vcpu to allow movement to another core or the
task to load its own state.

Signed-off-by: Andrew Scull <ascull@google.com>
Cc: Dave Martin <Dave.Martin@arm.com>
---
 arch/arm64/include/asm/fpsimd.h |  1 +
 arch/arm64/kernel/fpsimd.c      | 11 +++++++++--
 arch/arm64/kvm/fpsimd.c         | 18 ++++++++++++++++--
 3 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
index bec5f14b622a..fc0b932211f0 100644
--- a/arch/arm64/include/asm/fpsimd.h
+++ b/arch/arm64/include/asm/fpsimd.h
@@ -48,6 +48,7 @@ extern void fpsimd_update_current_state(struct user_fpsimd_state const *state);
 extern void fpsimd_bind_task_to_cpu(void);
 extern void fpsimd_bind_state_to_cpu(struct user_fpsimd_state *state,
 				     void *sve_state, unsigned int sve_vl);
+extern bool fpsimd_is_bound_to_cpu(struct user_fpsimd_state *state);
 
 extern void fpsimd_flush_task_state(struct task_struct *target);
 extern void fpsimd_save_and_flush_cpu_state(void);
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 062b21f30f94..683675b5d198 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -1009,8 +1009,7 @@ void fpsimd_thread_switch(struct task_struct *next)
 	 * state.  For kernel threads, FPSIMD registers are never loaded
 	 * and wrong_task and wrong_cpu will always be true.
 	 */
-	wrong_task = __this_cpu_read(fpsimd_last_state.st) !=
-					&next->thread.uw.fpsimd_state;
+	wrong_task = !fpsimd_is_bound_to_cpu(&next->thread.uw.fpsimd_state);
 	wrong_cpu = next->thread.fpsimd_cpu != smp_processor_id();
 
 	update_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE,
@@ -1137,6 +1136,14 @@ void fpsimd_bind_state_to_cpu(struct user_fpsimd_state *st, void *sve_state,
 	last->sve_vl = sve_vl;
 }
 
+bool fpsimd_is_bound_to_cpu(struct user_fpsimd_state *st)
+{
+	WARN_ON(!system_supports_fpsimd());
+	WARN_ON(!in_softirq() && !irqs_disabled());
+
+	return __this_cpu_read(fpsimd_last_state.st) == st;
+}
+
 /*
  * Load the userland FPSIMD state of 'current' from memory, but only if the
  * FPSIMD state already held in the registers is /not/ the most recent FPSIMD
diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
index 3e5a02137643..dcc5bfad5408 100644
--- a/arch/arm64/kvm/fpsimd.c
+++ b/arch/arm64/kvm/fpsimd.c
@@ -51,6 +51,8 @@ int kvm_arch_vcpu_run_map_fp(struct kvm_vcpu *vcpu)
  */
 void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
 {
+	unsigned long flags;
+
 	BUG_ON(!current->mm);
 
 	vcpu->arch.flags &= ~(KVM_ARM64_FP_ENABLED |
@@ -61,13 +63,25 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
 	if (!system_supports_fpsimd())
 		return;
 
-	vcpu->arch.flags |= KVM_ARM64_FP_HOST;
+	local_irq_save(flags);
+
+	if (fpsimd_is_bound_to_cpu(&vcpu->arch.ctxt.fp_regs) &&
+	    vcpu->arch.fpsimd_cpu == smp_processor_id()) {
+		clear_thread_flag(TIF_FOREIGN_FPSTATE);
+		update_thread_flag(TIF_SVE, vcpu_has_sve(vcpu));
+
+		vcpu->arch.flags |= KVM_ARM64_FP_ENABLED;
+	} else {
+		vcpu->arch.flags |= KVM_ARM64_FP_HOST;
+	}
 
 	if (test_thread_flag(TIF_SVE))
 		vcpu->arch.flags |= KVM_ARM64_HOST_SVE_IN_USE;
 
 	if (read_sysreg(cpacr_el1) & CPACR_EL1_ZEN_EL0EN)
 		vcpu->arch.flags |= KVM_ARM64_HOST_SVE_ENABLED;
+
+	local_irq_restore(flags);
 }
 
 
@@ -124,7 +138,7 @@ void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
 	local_irq_save(flags);
 
 	if (vcpu->arch.flags & KVM_ARM64_FP_ENABLED) {
-		fpsimd_save_and_flush_cpu_state();
+		fpsimd_thread_switch(current);
 
 		if (guest_has_sve)
 			__vcpu_sys_reg(vcpu, ZCR_EL1) = read_sysreg_s(SYS_ZCR_EL12);
-- 
2.30.1.766.gb4fecdf3b7-goog

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