MIPS: KVM: Add base guest FPU support
Add base code for supporting FPU in MIPS KVM guests. The FPU cannot yet
be enabled in the guest, we're just laying the groundwork.
Whether the guest's FPU context is loaded is stored in a bit in the
fpu_inuse vcpu member. This allows the FPU to be disabled when the guest
disables it, but keeping the FPU context loaded so it doesn't have to be
reloaded if the guest re-enables it.
An fpu_enabled vcpu member stores whether userland has enabled the FPU
capability (which will be wired up in a later patch).
New assembly code is added for saving and restoring the FPU context, and
for saving/clearing and restoring FCSR (which can itself cause an FP
exception depending on the value). The FCSR is restored before returning
to the guest if the FPU is already enabled, and a die notifier is
registered to catch the possible FP exception and step over the ctc1
instruction.
The helper function kvm_lose_fpu() is added to save FPU context and
disable the FPU, which is used when saving hardware state before a
context switch or KVM exit (the vcpu_get_regs() callback).
The helper function kvm_own_fpu() is added to enable the FPU and restore
the FPU context if it isn't already loaded, which will be used in a
later patch when the guest attempts to use the FPU for the first time
and triggers a co-processor unusable exception.
The helper function kvm_drop_fpu() is added to discard the FPU context
and disable the FPU, which will be used in a later patch when the FPU
state will become architecturally UNPREDICTABLE (change of FR mode) to
force a reload of [stale] context in the new FR mode.
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Gleb Natapov <gleb@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org
diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
index 73eecc7..b26a48d 100644
--- a/arch/mips/kvm/mips.c
+++ b/arch/mips/kvm/mips.c
@@ -11,6 +11,7 @@
#include <linux/errno.h>
#include <linux/err.h>
+#include <linux/kdebug.h>
#include <linux/module.h>
#include <linux/vmalloc.h>
#include <linux/fs.h>
@@ -1178,12 +1179,133 @@
}
}
+ if (ret == RESUME_GUEST) {
+ /*
+ * If FPU is enabled (i.e. the guest's FPU context is live),
+ * restore FCR31.
+ *
+ * This should be before returning to the guest exception
+ * vector, as it may well cause an FP exception if there are
+ * pending exception bits unmasked. (see
+ * kvm_mips_csr_die_notifier() for how that is handled).
+ */
+ if (kvm_mips_guest_has_fpu(&vcpu->arch) &&
+ read_c0_status() & ST0_CU1)
+ __kvm_restore_fcsr(&vcpu->arch);
+ }
+
/* Disable HTW before returning to guest or host */
htw_stop();
return ret;
}
+/* Enable FPU for guest and restore context */
+void kvm_own_fpu(struct kvm_vcpu *vcpu)
+{
+ struct mips_coproc *cop0 = vcpu->arch.cop0;
+ unsigned int sr, cfg5;
+
+ preempt_disable();
+
+ /*
+ * Enable FPU for guest
+ * We set FR and FRE according to guest context
+ */
+ sr = kvm_read_c0_guest_status(cop0);
+ change_c0_status(ST0_CU1 | ST0_FR, sr);
+ if (cpu_has_fre) {
+ cfg5 = kvm_read_c0_guest_config5(cop0);
+ change_c0_config5(MIPS_CONF5_FRE, cfg5);
+ }
+ enable_fpu_hazard();
+
+ /* If guest FPU state not active, restore it now */
+ if (!(vcpu->arch.fpu_inuse & KVM_MIPS_FPU_FPU)) {
+ __kvm_restore_fpu(&vcpu->arch);
+ vcpu->arch.fpu_inuse |= KVM_MIPS_FPU_FPU;
+ }
+
+ preempt_enable();
+}
+
+/* Drop FPU without saving it */
+void kvm_drop_fpu(struct kvm_vcpu *vcpu)
+{
+ preempt_disable();
+ if (vcpu->arch.fpu_inuse & KVM_MIPS_FPU_FPU) {
+ clear_c0_status(ST0_CU1 | ST0_FR);
+ vcpu->arch.fpu_inuse &= ~KVM_MIPS_FPU_FPU;
+ }
+ preempt_enable();
+}
+
+/* Save and disable FPU */
+void kvm_lose_fpu(struct kvm_vcpu *vcpu)
+{
+ /*
+ * FPU gets disabled in root context (hardware) when it is disabled in
+ * guest context (software), but the register state in the hardware may
+ * still be in use. This is why we explicitly re-enable the hardware
+ * before saving.
+ */
+
+ preempt_disable();
+ if (vcpu->arch.fpu_inuse & KVM_MIPS_FPU_FPU) {
+ set_c0_status(ST0_CU1);
+ enable_fpu_hazard();
+
+ __kvm_save_fpu(&vcpu->arch);
+ vcpu->arch.fpu_inuse &= ~KVM_MIPS_FPU_FPU;
+
+ /* Disable FPU */
+ clear_c0_status(ST0_CU1 | ST0_FR);
+ }
+ preempt_enable();
+}
+
+/*
+ * Step over a specific ctc1 to FCSR which is used to restore guest FCSR state
+ * and may trigger a "harmless" FP exception if cause bits are set in the value
+ * being written.
+ */
+static int kvm_mips_csr_die_notify(struct notifier_block *self,
+ unsigned long cmd, void *ptr)
+{
+ struct die_args *args = (struct die_args *)ptr;
+ struct pt_regs *regs = args->regs;
+ unsigned long pc;
+
+ /* Only interested in FPE */
+ if (cmd != DIE_FP)
+ return NOTIFY_DONE;
+
+ /* Return immediately if guest context isn't active */
+ if (!(current->flags & PF_VCPU))
+ return NOTIFY_DONE;
+
+ /* Should never get here from user mode */
+ BUG_ON(user_mode(regs));
+
+ pc = instruction_pointer(regs);
+ switch (cmd) {
+ case DIE_FP:
+ /* match 2nd instruction in __kvm_restore_fcsr */
+ if (pc != (unsigned long)&__kvm_restore_fcsr + 4)
+ return NOTIFY_DONE;
+ break;
+ }
+
+ /* Move PC forward a little and continue executing */
+ instruction_pointer(regs) += 4;
+
+ return NOTIFY_STOP;
+}
+
+static struct notifier_block kvm_mips_csr_die_notifier = {
+ .notifier_call = kvm_mips_csr_die_notify,
+};
+
int __init kvm_mips_init(void)
{
int ret;
@@ -1193,6 +1315,8 @@
if (ret)
return ret;
+ register_die_notifier(&kvm_mips_csr_die_notifier);
+
/*
* On MIPS, kernel modules are executed from "mapped space", which
* requires TLBs. The TLB handling code is statically linked with
@@ -1215,6 +1339,8 @@
kvm_mips_gfn_to_pfn = NULL;
kvm_mips_release_pfn_clean = NULL;
kvm_mips_is_error_pfn = NULL;
+
+ unregister_die_notifier(&kvm_mips_csr_die_notifier);
}
module_init(kvm_mips_init);