x86/hvm/viridian: keep APIC assist page mapped...

... for the lifetime of the domain.

If Xen is to make use of the APIC assist enlightenment then a persistent
mapping needs to be kept, rather than the temporary one which is currently
used only to initialize the page content.

This patch also adds a comment block at the top of the source with
information on the latest version of the spec. from Microsoft and the
current URL where it may be found.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/hvm/hvm.c

@@ -2606,6 +2606,8 @@ int hvm_vcpu_initialise(struct vcpu *v)
 
 void hvm_vcpu_destroy(struct vcpu *v)
 {
+    viridian_vcpu_deinit(v);
+
     hvm_all_ioreq_servers_remove_vcpu(v->domain, v);
 
     if ( hvm_altp2m_supported() )

xen/arch/x86/hvm/viridian.c

@@ -1,7 +1,12 @@
 /******************************************************************************
  * viridian.c
  *
- * An implementation of the Viridian hypercall interface.
+ * An implementation of some Viridian enlightenments. See Microsoft's
+ * Hypervisor Top Level Functional Specification (v4.0b) at:
+ *
+ * https://msdn.microsoft.com/en-us/virtualization/hyperv_on_windows/develop/tlfs 
+ *
+ * for more information.
  */
 
 #include <xen/sched.h>
@@ -163,7 +168,7 @@ static void dump_apic_assist(const struct vcpu *v)
 {
     const union viridian_apic_assist *aa;
 
-    aa = &v->arch.hvm_vcpu.viridian.apic_assist;
+    aa = &v->arch.hvm_vcpu.viridian.apic_assist.msr;
 
     printk(XENLOG_G_INFO "%pv: VIRIDIAN APIC_ASSIST: enabled: %x pfn: %lx\n",
            v, aa->fields.enabled, (unsigned long)aa->fields.pfn);
@@ -217,9 +222,9 @@ static void enable_hypercall_page(struct domain *d)
 static void initialize_apic_assist(struct vcpu *v)
 {
     struct domain *d = v->domain;
-    unsigned long gmfn = v->arch.hvm_vcpu.viridian.apic_assist.fields.pfn;
+    unsigned long gmfn = v->arch.hvm_vcpu.viridian.apic_assist.msr.fields.pfn;
     struct page_info *page = get_page_from_gfn(d, gmfn, NULL, P2M_ALLOC);
-    uint8_t *p;
+    void *va;
 
     /*
      * We don't yet make use of the APIC assist page but by setting
@@ -227,25 +232,49 @@ static void initialize_apic_assist(struct vcpu *v)
      * bound to support the MSR. We therefore do just enough to keep windows
      * happy.
      *
-     * See http://msdn.microsoft.com/en-us/library/ff538657%28VS.85%29.aspx for
-     * details of how Windows uses the page.
+     * See section 13.3.4.1 of the specification for details of this
+     * enlightenment.
      */
 
-    if ( !page || !get_page_type(page, PGT_writable_page) )
+    if ( !page )
+        goto fail;
+
+    if ( !get_page_type(page, PGT_writable_page) )
     {
-        if ( page )
-            put_page(page);
-        gdprintk(XENLOG_WARNING, "Bad GMFN %lx (MFN %lx)\n", gmfn,
-                 page ? page_to_mfn(page) : INVALID_MFN);
-        return;
+        put_page(page);
+        goto fail;
     }
 
-    p = __map_domain_page(page);
+    va = __map_domain_page_global(page);
+    if ( !va )
+    {
+        put_page_and_type(page);
+        goto fail;
+    }
 
-    *(u32 *)p = 0;
+    *(uint32_t *)va = 0;
 
-    unmap_domain_page(p);
+    v->arch.hvm_vcpu.viridian.apic_assist.va = va;
+    return;
 
+ fail:
+    gdprintk(XENLOG_WARNING, "Bad GMFN %lx (MFN %lx)\n", gmfn,
+             page ? page_to_mfn(page) : INVALID_MFN);
+}
+
+static void teardown_apic_assist(struct vcpu *v)
+{
+    void *va = v->arch.hvm_vcpu.viridian.apic_assist.va;
+    struct page_info *page;
+
+    if ( !va )
+        return;
+
+    v->arch.hvm_vcpu.viridian.apic_assist.va = NULL;
+
+    page = mfn_to_page(domain_page_map_to_mfn(va));
+
+    unmap_domain_page_global(va);
     put_page_and_type(page);
 }
 
@@ -374,9 +403,10 @@ int wrmsr_viridian_regs(uint32_t idx, uint64_t val)
 
     case VIRIDIAN_MSR_APIC_ASSIST:
         perfc_incr(mshv_wrmsr_apic_msr);
-        v->arch.hvm_vcpu.viridian.apic_assist.raw = val;
+        teardown_apic_assist(v); /* release any previous mapping */
+        v->arch.hvm_vcpu.viridian.apic_assist.msr.raw = val;
         dump_apic_assist(v);
-        if (v->arch.hvm_vcpu.viridian.apic_assist.fields.enabled)
+        if ( v->arch.hvm_vcpu.viridian.apic_assist.msr.fields.enabled )
             initialize_apic_assist(v);
         break;
 
@@ -485,7 +515,7 @@ int rdmsr_viridian_regs(uint32_t idx, uint64_t *val)
 
     case VIRIDIAN_MSR_APIC_ASSIST:
         perfc_incr(mshv_rdmsr_apic_msr);
-        *val = v->arch.hvm_vcpu.viridian.apic_assist.raw;
+        *val = v->arch.hvm_vcpu.viridian.apic_assist.msr.raw;
         break;
 
     case VIRIDIAN_MSR_REFERENCE_TSC:
@@ -521,6 +551,11 @@ int rdmsr_viridian_regs(uint32_t idx, uint64_t *val)
     return 1;
 }
 
+void viridian_vcpu_deinit(struct vcpu *v)
+{
+    teardown_apic_assist(v);
+}
+
 static DEFINE_PER_CPU(cpumask_t, ipi_cpumask);
 
 int viridian_hypercall(struct cpu_user_regs *regs)
@@ -721,7 +756,7 @@ static int viridian_save_vcpu_ctxt(struct domain *d, hvm_domain_context_t *h)
     for_each_vcpu( d, v ) {
         struct hvm_viridian_vcpu_context ctxt;
 
-        ctxt.apic_assist = v->arch.hvm_vcpu.viridian.apic_assist.raw;
+        ctxt.apic_assist = v->arch.hvm_vcpu.viridian.apic_assist.msr.raw;
 
         if ( hvm_save_entry(VIRIDIAN_VCPU, v->vcpu_id, h, &ctxt) != 0 )
             return 1;
@@ -747,7 +782,9 @@ static int viridian_load_vcpu_ctxt(struct domain *d, hvm_domain_context_t *h)
     if ( hvm_load_entry(VIRIDIAN_VCPU, h, &ctxt) != 0 )
         return -EINVAL;
 
-    v->arch.hvm_vcpu.viridian.apic_assist.raw = ctxt.apic_assist;
+    v->arch.hvm_vcpu.viridian.apic_assist.msr.raw = ctxt.apic_assist;
+    if ( v->arch.hvm_vcpu.viridian.apic_assist.msr.fields.enabled )
+        initialize_apic_assist(v);
 
     return 0;
 }

xen/include/asm-x86/hvm/viridian.h

@@ -21,7 +21,10 @@ union viridian_apic_assist
 
 struct viridian_vcpu
 {
-    union viridian_apic_assist apic_assist;
+    struct {
+        union viridian_apic_assist msr;
+        void *va;
+    } apic_assist;
 };
 
 union viridian_guest_os_id
@@ -117,6 +120,8 @@ viridian_hypercall(struct cpu_user_regs *regs);
 void viridian_time_ref_count_freeze(struct domain *d);
 void viridian_time_ref_count_thaw(struct domain *d);
 
+void viridian_vcpu_deinit(struct vcpu *v);
+
 #endif /* __ASM_X86_HVM_VIRIDIAN_H__ */
 
 /*