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[PATCH 6/10] KVM: MIPS/T&E: Add lockless GVA access helpers

To: <linux-mips@linux-mips.org>
Subject: [PATCH 6/10] KVM: MIPS/T&E: Add lockless GVA access helpers
From: James Hogan <james.hogan@imgtec.com>
Date: Mon, 9 Jan 2017 20:51:58 +0000
Cc: James Hogan <james.hogan@imgtec.com>, Paolo Bonzini <pbonzini@redhat.com>, Radim Krčmář <rkrcmar@redhat.com>, Ralf Baechle <ralf@linux-mips.org>, <kvm@vger.kernel.org>
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Add helpers to allow for lockless direct access to the GVA space, by
changing the VCPU mode to READING_SHADOW_PAGE_TABLES for the duration of
the access. This allows asynchronous TLB flush requests in future
patches to safely trigger either a TLB flush before the direct GVA space
access, or a delay until the in-progress lockless direct access is
complete.

The kvm_trap_emul_gva_lockless_begin() and
kvm_trap_emul_gva_lockless_end() helpers take care of guarding the
direct GVA accesses, and kvm_trap_emul_gva_fault() tries to handle a
uaccess fault resulting from a flush having taken place.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org
---
 arch/mips/include/asm/kvm_host.h | 15 ++++++++-
 arch/mips/kvm/mmu.c              | 51 ++++++++++++++++++++++++++-
 arch/mips/kvm/trap_emul.c        | 65 +++++++++++++++++++++++++++++++++-
 3 files changed, 131 insertions(+), 0 deletions(-)

diff --git a/arch/mips/include/asm/kvm_host.h b/arch/mips/include/asm/kvm_host.h
index b2d968ed2757..491689fcbaac 100644
--- a/arch/mips/include/asm/kvm_host.h
+++ b/arch/mips/include/asm/kvm_host.h
@@ -243,6 +243,7 @@ enum emulation_result {
 #define TLB_ASID(x)            ((x).tlb_hi & KVM_ENTRYHI_ASID)
 #define TLB_LO_IDX(x, va)      (((va) >> PAGE_SHIFT) & 1)
 #define TLB_IS_VALID(x, va)    ((x).tlb_lo[TLB_LO_IDX(x, va)] & ENTRYLO_V)
+#define TLB_IS_DIRTY(x, va)    ((x).tlb_lo[TLB_LO_IDX(x, va)] & ENTRYLO_D)
 #define TLB_HI_VPN2_HIT(x, y)  ((TLB_VPN2(x) & ~(x).tlb_mask) ==       \
                                 ((y) & VPN2_MASK & ~(x).tlb_mask))
 #define TLB_HI_ASID_HIT(x, y)  (TLB_IS_GLOBAL(x) ||                    \
@@ -637,6 +638,20 @@ pgd_t *kvm_pgd_alloc(void);
 void kvm_mmu_free_memory_caches(struct kvm_vcpu *vcpu);
 void kvm_trap_emul_invalidate_gva(struct kvm_vcpu *vcpu, unsigned long addr,
                                  bool user);
+void kvm_trap_emul_gva_lockless_begin(struct kvm_vcpu *vcpu);
+void kvm_trap_emul_gva_lockless_end(struct kvm_vcpu *vcpu);
+
+enum kvm_mips_fault_result {
+       KVM_MIPS_MAPPED = 0,
+       KVM_MIPS_GVA,
+       KVM_MIPS_GPA,
+       KVM_MIPS_TLB,
+       KVM_MIPS_TLBINV,
+       KVM_MIPS_TLBMOD,
+};
+enum kvm_mips_fault_result kvm_trap_emul_gva_fault(struct kvm_vcpu *vcpu,
+                                                  unsigned long gva,
+                                                  bool write);
 
 /* Emulation */
 int kvm_get_inst(u32 *opc, struct kvm_vcpu *vcpu, u32 *out);
diff --git a/arch/mips/kvm/mmu.c b/arch/mips/kvm/mmu.c
index e41ee36dd626..32c317de6c0a 100644
--- a/arch/mips/kvm/mmu.c
+++ b/arch/mips/kvm/mmu.c
@@ -732,6 +732,57 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
        local_irq_restore(flags);
 }
 
+/**
+ * kvm_trap_emul_gva_fault() - Safely attempt to handle a GVA access fault.
+ * @vcpu:      Virtual CPU.
+ * @gva:       Guest virtual address to be accessed.
+ * @write:     True if write attempted (must be dirtied and made writable).
+ *
+ * Safely attempt to handle a GVA fault, mapping GVA pages if necessary, and
+ * dirtying the page if @write so that guest instructions can be modified.
+ *
+ * Returns:    KVM_MIPS_MAPPED on success.
+ *             KVM_MIPS_GVA if bad guest virtual address.
+ *             KVM_MIPS_GPA if bad guest physical address.
+ *             KVM_MIPS_TLB if guest TLB not present.
+ *             KVM_MIPS_TLBINV if guest TLB present but not valid.
+ *             KVM_MIPS_TLBMOD if guest TLB read only.
+ */
+enum kvm_mips_fault_result kvm_trap_emul_gva_fault(struct kvm_vcpu *vcpu,
+                                                  unsigned long gva,
+                                                  bool write)
+{
+       struct mips_coproc *cop0 = vcpu->arch.cop0;
+       struct kvm_mips_tlb *tlb;
+       int index;
+
+       if (KVM_GUEST_KSEGX(gva) == KVM_GUEST_KSEG0) {
+               if (kvm_mips_handle_kseg0_tlb_fault(gva, vcpu) < 0)
+                       return KVM_MIPS_GPA;
+       } else if ((KVM_GUEST_KSEGX(gva) < KVM_GUEST_KSEG0) ||
+                  KVM_GUEST_KSEGX(gva) == KVM_GUEST_KSEG23) {
+               /* Address should be in the guest TLB */
+               index = kvm_mips_guest_tlb_lookup(vcpu, (gva & VPN2_MASK) |
+                         (kvm_read_c0_guest_entryhi(cop0) & KVM_ENTRYHI_ASID));
+               if (index < 0)
+                       return KVM_MIPS_TLB;
+               tlb = &vcpu->arch.guest_tlb[index];
+
+               /* Entry should be valid, and dirty for writes */
+               if (!TLB_IS_VALID(*tlb, gva))
+                       return KVM_MIPS_TLBINV;
+               if (write && !TLB_IS_DIRTY(*tlb, gva))
+                       return KVM_MIPS_TLBMOD;
+
+               if (kvm_mips_handle_mapped_seg_tlb_fault(vcpu, tlb, gva))
+                       return KVM_MIPS_GPA;
+       } else {
+               return KVM_MIPS_GVA;
+       }
+
+       return KVM_MIPS_MAPPED;
+}
+
 int kvm_get_inst(u32 *opc, struct kvm_vcpu *vcpu, u32 *out)
 {
        int err;
diff --git a/arch/mips/kvm/trap_emul.c b/arch/mips/kvm/trap_emul.c
index 0cb76e1aac0e..2cc637ed9e95 100644
--- a/arch/mips/kvm/trap_emul.c
+++ b/arch/mips/kvm/trap_emul.c
@@ -788,6 +788,71 @@ static void kvm_trap_emul_check_requests(struct kvm_vcpu 
*vcpu, int cpu,
        }
 }
 
+/**
+ * kvm_trap_emul_gva_lockless_begin() - Begin lockless access to GVA space.
+ * @vcpu:      VCPU pointer.
+ *
+ * Call before a GVA space access outside of guest mode, to ensure that
+ * asynchronous TLB flush requests are handled or delayed until completion of
+ * the GVA access (as indicated by a matching 
kvm_trap_emul_gva_lockless_end()).
+ *
+ * Should be called with IRQs already enabled.
+ */
+void kvm_trap_emul_gva_lockless_begin(struct kvm_vcpu *vcpu)
+{
+       /* We re-enable IRQs in kvm_trap_emul_gva_lockless_end() */
+       WARN_ON_ONCE(irqs_disabled());
+
+       /*
+        * The caller is about to access the GVA space, so we set the mode to
+        * force TLB flush requests to send an IPI, and also disable IRQs to
+        * delay IPI handling until kvm_trap_emul_gva_lockless_end().
+        */
+       local_irq_disable();
+
+       /*
+        * Make sure the read of VCPU requests is not reordered ahead of the
+        * write to vcpu->mode, or we could miss a TLB flush request while
+        * the requester sees the VCPU as outside of guest mode and not needing
+        * an IPI.
+        */
+       smp_store_mb(vcpu->mode, READING_SHADOW_PAGE_TABLES);
+
+       /*
+        * If a TLB flush has been requested (potentially while
+        * OUTSIDE_GUEST_MODE and assumed immediately effective), perform it
+        * before accessing the GVA space, and be sure to reload the ASID if
+        * necessary as it'll be immediately used.
+        *
+        * TLB flush requests after this check will trigger an IPI due to the
+        * mode change above, which will be delayed due to IRQs disabled.
+        */
+       kvm_trap_emul_check_requests(vcpu, smp_processor_id(), true);
+}
+
+/**
+ * kvm_trap_emul_gva_lockless_end() - End lockless access to GVA space.
+ * @vcpu:      VCPU pointer.
+ *
+ * Called after a GVA space access outside of guest mode. Should have a 
matching
+ * call to kvm_trap_emul_gva_lockless_begin().
+ */
+void kvm_trap_emul_gva_lockless_end(struct kvm_vcpu *vcpu)
+{
+       /*
+        * Make sure the write to vcpu->mode is not reordered in front of GVA
+        * accesses, or a TLB flush requester may not think it necessary to send
+        * an IPI.
+        */
+       smp_store_release(&vcpu->mode, OUTSIDE_GUEST_MODE);
+
+       /*
+        * Now that the access to GVA space is complete, its safe for pending
+        * TLB flush request IPIs to be handled (which indicates completion).
+        */
+       local_irq_enable();
+}
+
 static void kvm_trap_emul_vcpu_reenter(struct kvm_run *run,
                                       struct kvm_vcpu *vcpu)
 {
-- 
git-series 0.8.10

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