MIPS: KVM: Add base guest MSA support

Add base code for supporting the MIPS SIMD Architecture (MSA) in MIPS
KVM guests. MSA cannot yet be enabled in the guest, we're just laying
the groundwork.

As with the FPU, whether the guest's MSA context is loaded is stored in
another bit in the fpu_inuse vcpu member. This allows MSA to be disabled
when the guest disables it, but keeping the MSA context loaded so it
doesn't have to be reloaded if the guest re-enables it.

New assembly code is added for saving and restoring the MSA context,
restoring only the upper half of the MSA context (for if the FPU context
is already loaded) and for saving/clearing and restoring MSACSR (which
can itself cause an MSA FP exception depending on the value). The MSACSR
is restored before returning to the guest if MSA is already enabled, and
the existing FP exception die notifier is extended to catch the possible
MSA FP exception and step over the ctcmsa instruction.

The helper function kvm_own_msa() is added to enable MSA and restore
the MSA context if it isn't already loaded, which will be used in a
later patch when the guest attempts to use MSA for the first time and
triggers an MSA disabled exception.

The existing FPU helpers are extended to handle MSA. kvm_lose_fpu()
saves the full MSA context if it is loaded (which includes the FPU
context) and both kvm_lose_fpu() and kvm_drop_fpu() disable MSA.

kvm_own_fpu() also needs to lose any MSA context if FR=0, since there
would be a risk of getting reserved instruction exceptions if CU1 is
enabled and we later try and save the MSA context. We shouldn't usually
hit this case since it will be handled when emulating CU1 changes,
however there's nothing to stop the guest modifying the Status register
directly via the comm page, which will cause this case to get hit.

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

1 files changed, 116 insertions, 16 deletions

diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
index 7f86cb73d05d..a17f21015a0b 100644
--- a/arch/mips/kvm/mips.c
+++ b/arch/mips/kvm/mips.c
@@ -1295,17 +1295,21 @@ skip_emul:
 
         if (ret == RESUME_GUEST) {
                 /*
-                 * If FPU is enabled (i.e. the guest's FPU context is live),
-                 * restore FCR31.
+                 * If FPU / MSA are enabled (i.e. the guest's FPU / MSA context
+                 * is live), restore FCR31 / MSACSR.
                  *
                  * 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
+                 * vector, as it may well cause an [MSA] 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);
+
+                if (kvm_mips_guest_has_msa(&vcpu->arch) &&
+                    read_c0_config5() & MIPS_CONF5_MSAEN)
+                        __kvm_restore_msacsr(&vcpu->arch);
         }
 
         /* Disable HTW before returning to guest or host */
@@ -1322,11 +1326,26 @@ void kvm_own_fpu(struct kvm_vcpu *vcpu)
 
         preempt_disable();
 
+        sr = kvm_read_c0_guest_status(cop0);
+
+        /*
+         * If MSA state is already live, it is undefined how it interacts with
+         * FR=0 FPU state, and we don't want to hit reserved instruction
+         * exceptions trying to save the MSA state later when CU=1 && FR=1, so
+         * play it safe and save it first.
+         *
+         * In theory we shouldn't ever hit this case since kvm_lose_fpu() should
+         * get called when guest CU1 is set, however we can't trust the guest
+         * not to clobber the status register directly via the commpage.
+         */
+        if (cpu_has_msa && sr & ST0_CU1 && !(sr & ST0_FR) &&
+            vcpu->arch.fpu_inuse & KVM_MIPS_FPU_MSA)
+                kvm_lose_fpu(vcpu);
+
         /*
          * 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);
@@ -1343,10 +1362,73 @@ void kvm_own_fpu(struct kvm_vcpu *vcpu)
         preempt_enable();
 }
 
-/* Drop FPU without saving it */
+#ifdef CONFIG_CPU_HAS_MSA
+/* Enable MSA for guest and restore context */
+void kvm_own_msa(struct kvm_vcpu *vcpu)
+{
+        struct mips_coproc *cop0 = vcpu->arch.cop0;
+        unsigned int sr, cfg5;
+
+        preempt_disable();
+
+        /*
+         * Enable FPU if enabled in guest, since we're restoring FPU context
+         * anyway. We set FR and FRE according to guest context.
+         */
+        if (kvm_mips_guest_has_fpu(&vcpu->arch)) {
+                sr = kvm_read_c0_guest_status(cop0);
+
+                /*
+                 * If FR=0 FPU state is already live, it is undefined how it
+                 * interacts with MSA state, so play it safe and save it first.
+                 */
+                if (!(sr & ST0_FR) &&
+                    (vcpu->arch.fpu_inuse & (KVM_MIPS_FPU_FPU |
+                                KVM_MIPS_FPU_MSA)) == KVM_MIPS_FPU_FPU)
+                        kvm_lose_fpu(vcpu);
+
+                change_c0_status(ST0_CU1 | ST0_FR, sr);
+                if (sr & ST0_CU1 && cpu_has_fre) {
+                        cfg5 = kvm_read_c0_guest_config5(cop0);
+                        change_c0_config5(MIPS_CONF5_FRE, cfg5);
+                }
+        }
+
+        /* Enable MSA for guest */
+        set_c0_config5(MIPS_CONF5_MSAEN);
+        enable_fpu_hazard();
+
+        switch (vcpu->arch.fpu_inuse & (KVM_MIPS_FPU_FPU | KVM_MIPS_FPU_MSA)) {
+        case KVM_MIPS_FPU_FPU:
+                /*
+                 * Guest FPU state already loaded, only restore upper MSA state
+                 */
+                __kvm_restore_msa_upper(&vcpu->arch);
+                vcpu->arch.fpu_inuse |= KVM_MIPS_FPU_MSA;
+                break;
+        case 0:
+                /* Neither FPU or MSA already active, restore full MSA state */
+                __kvm_restore_msa(&vcpu->arch);
+                vcpu->arch.fpu_inuse |= KVM_MIPS_FPU_MSA;
+                if (kvm_mips_guest_has_fpu(&vcpu->arch))
+                        vcpu->arch.fpu_inuse |= KVM_MIPS_FPU_FPU;
+                break;
+        default:
+                break;
+        }
+
+        preempt_enable();
+}
+#endif
+
+/* Drop FPU & MSA without saving it */
 void kvm_drop_fpu(struct kvm_vcpu *vcpu)
 {
         preempt_disable();
+        if (cpu_has_msa && vcpu->arch.fpu_inuse & KVM_MIPS_FPU_MSA) {
+                disable_msa();
+                vcpu->arch.fpu_inuse &= ~KVM_MIPS_FPU_MSA;
+        }
         if (vcpu->arch.fpu_inuse & KVM_MIPS_FPU_FPU) {
                 clear_c0_status(ST0_CU1 | ST0_FR);
                 vcpu->arch.fpu_inuse &= ~KVM_MIPS_FPU_FPU;
@@ -1354,18 +1436,29 @@ void kvm_drop_fpu(struct kvm_vcpu *vcpu)
         preempt_enable();
 }
 
-/* Save and disable FPU */
+/* Save and disable FPU & MSA */
 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
+         * FPU & MSA get 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) {
+        if (cpu_has_msa && vcpu->arch.fpu_inuse & KVM_MIPS_FPU_MSA) {
+                set_c0_config5(MIPS_CONF5_MSAEN);
+                enable_fpu_hazard();
+
+                __kvm_save_msa(&vcpu->arch);
+
+                /* Disable MSA & FPU */
+                disable_msa();
+                if (vcpu->arch.fpu_inuse & KVM_MIPS_FPU_FPU)
+                        clear_c0_status(ST0_CU1 | ST0_FR);
+                vcpu->arch.fpu_inuse &= ~(KVM_MIPS_FPU_FPU | KVM_MIPS_FPU_MSA);
+        } else if (vcpu->arch.fpu_inuse & KVM_MIPS_FPU_FPU) {
                 set_c0_status(ST0_CU1);
                 enable_fpu_hazard();
 
@@ -1379,9 +1472,9 @@ void kvm_lose_fpu(struct kvm_vcpu *vcpu)
 }
 
 /*
- * 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.
+ * Step over a specific ctc1 to FCSR and a specific ctcmsa to MSACSR which are
+ * used to restore guest FCSR/MSACSR state and may trigger a "harmless" FP/MSAFP
+ * 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)
@@ -1390,8 +1483,8 @@ static int kvm_mips_csr_die_notify(struct notifier_block *self,
         struct pt_regs *regs = args->regs;
         unsigned long pc;
 
-        /* Only interested in FPE */
-        if (cmd != DIE_FP)
+        /* Only interested in FPE and MSAFPE */
+        if (cmd != DIE_FP && cmd != DIE_MSAFP)
                 return NOTIFY_DONE;
 
         /* Return immediately if guest context isn't active */
@@ -1408,6 +1501,13 @@ static int kvm_mips_csr_die_notify(struct notifier_block *self,
                 if (pc != (unsigned long)&__kvm_restore_fcsr + 4)
                         return NOTIFY_DONE;
                 break;
+        case DIE_MSAFP:
+                /* match 2nd/3rd instruction in __kvm_restore_msacsr */
+                if (!cpu_has_msa ||
+                    pc < (unsigned long)&__kvm_restore_msacsr + 4 ||
+                    pc > (unsigned long)&__kvm_restore_msacsr + 8)
+                        return NOTIFY_DONE;
+                break;
         }
 
         /* Move PC forward a little and continue executing */