Merge branch kvm-arm64/fpsimd-doc into kvmarm-master/next

* kvm-arm64/fpsimd-doc:
  : .
  : FPSIMD documentation update, courtesy of Mark Brown
  : .
  arm64/fpsimd: Clarify the purpose of using last in fpsimd_save()
  KVM: arm64: Add some more comments in kvm_hyp_handle_fpsimd()
  KVM: arm64: Add comments for context flush and sync callbacks
Signed-off-by: NMarc Zyngier <maz@kernel.org>

arch/arm64/kernel/fpsimd.c

@@ -348,7 +348,13 @@ static void task_fpsimd_load(void)
 
 /*
  * Ensure FPSIMD/SVE storage in memory for the loaded context is up to
- * date with respect to the CPU registers.
+ * date with respect to the CPU registers. Note carefully that the
+ * current context is the context last bound to the CPU stored in
+ * last, if KVM is involved this may be the guest VM context rather
+ * than the host thread for the VM pointed to by current. This means
+ * that we must always reference the state storage via last rather
+ * than via current, other than the TIF_ flags which KVM will
+ * carefully maintain for us.
  */
 static void fpsimd_save(void)
 {

arch/arm64/kvm/fpsimd.c

@@ -84,6 +84,11 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
 		vcpu->arch.flags |= KVM_ARM64_HOST_SVE_ENABLED;
 }
 
+/*
+ * Called just before entering the guest once we are no longer
+ * preemptable. Syncs the host's TIF_FOREIGN_FPSTATE with the KVM
+ * mirror of the flag used by the hypervisor.
+ */
 void kvm_arch_vcpu_ctxflush_fp(struct kvm_vcpu *vcpu)
 {
 	if (test_thread_flag(TIF_FOREIGN_FPSTATE))
@@ -93,10 +98,11 @@ void kvm_arch_vcpu_ctxflush_fp(struct kvm_vcpu *vcpu)
 }
 
 /*
- * If the guest FPSIMD state was loaded, update the host's context
- * tracking data mark the CPU FPSIMD regs as dirty and belonging to vcpu
- * so that they will be written back if the kernel clobbers them due to
- * kernel-mode NEON before re-entry into the guest.
+ * Called just after exiting the guest. If the guest FPSIMD state
+ * was loaded, update the host's context tracking data mark the CPU
+ * FPSIMD regs as dirty and belonging to vcpu so that they will be
+ * written back if the kernel clobbers them due to kernel-mode NEON
+ * before re-entry into the guest.
  */
 void kvm_arch_vcpu_ctxsync_fp(struct kvm_vcpu *vcpu)
 {

arch/arm64/kvm/hyp/include/hyp/switch.h

@@ -173,6 +173,8 @@ static bool kvm_hyp_handle_fpsimd(struct kvm_vcpu *vcpu, u64 *exit_code)
 		return false;
 
 	/* Valid trap.  Switch the context: */
+
+	/* First disable enough traps to allow us to update the registers */
 	if (has_vhe()) {
 		reg = CPACR_EL1_FPEN;
 		if (sve_guest)
@@ -188,11 +190,13 @@ static bool kvm_hyp_handle_fpsimd(struct kvm_vcpu *vcpu, u64 *exit_code)
 	}
 	isb();
 
+	/* Write out the host state if it's in the registers */
 	if (vcpu->arch.flags & KVM_ARM64_FP_HOST) {
 		__fpsimd_save_state(vcpu->arch.host_fpsimd_state);
 		vcpu->arch.flags &= ~KVM_ARM64_FP_HOST;
 	}
 
+	/* Restore the guest state */
 	if (sve_guest)
 		__hyp_sve_restore_guest(vcpu);
 	else