diff --git a/xen/arch/x86/e820.c b/xen/arch/x86/e820.c
index 082f9928a1..5a4ef27a0b 100644
--- a/xen/arch/x86/e820.c
+++ b/xen/arch/x86/e820.c
@@ -36,6 +36,22 @@ boolean_param("e820-verbose", e820_verbose);
 struct e820map e820;
 struct e820map __initdata e820_raw;
 
+static unsigned int find_phys_addr_bits(void)
+{
+    uint32_t eax;
+    unsigned int phys_bits = 36;
+
+    eax = cpuid_eax(0x80000000);
+    if ( (eax >> 16) == 0x8000 && eax >= 0x80000008 )
+    {
+        phys_bits = (uint8_t)cpuid_eax(0x80000008);
+        if ( phys_bits > PADDR_BITS )
+            phys_bits = PADDR_BITS;
+    }
+
+    return phys_bits;
+}
+
 /*
  * This function checks if the entire range <start,end> is mapped with type.
  *
@@ -357,6 +373,21 @@ static unsigned long __init find_max_pfn(void)
             max_pfn = end;
     }
 
+#ifdef CONFIG_HYPERV_GUEST
+    {
+	/*
+	 * We reserve the top-most page for hypercall page. Adjust
+	 * max_pfn if necessary.
+	 */
+        unsigned int phys_bits = find_phys_addr_bits();
+        unsigned long hcall_pfn =
+          ((1ull << phys_bits) - 1) >> PAGE_SHIFT;
+
+        if ( max_pfn >= hcall_pfn )
+          max_pfn = hcall_pfn - 1;
+    }
+#endif
+
     return max_pfn;
 }
 
@@ -420,7 +451,7 @@ static uint64_t __init mtrr_top_of_ram(void)
 {
     uint32_t eax, ebx, ecx, edx;
     uint64_t mtrr_cap, mtrr_def, addr_mask, base, mask, top;
-    unsigned int i, phys_bits = 36;
+    unsigned int i, phys_bits;
 
     /* By default we check only Intel systems. */
     if ( e820_mtrr_clip == -1 )
@@ -446,13 +477,7 @@ static uint64_t __init mtrr_top_of_ram(void)
          return 0;
 
     /* Find the physical address size for this CPU. */
-    eax = cpuid_eax(0x80000000);
-    if ( (eax >> 16) == 0x8000 && eax >= 0x80000008 )
-    {
-        phys_bits = (uint8_t)cpuid_eax(0x80000008);
-        if ( phys_bits > PADDR_BITS )
-            phys_bits = PADDR_BITS;
-    }
+    phys_bits = find_phys_addr_bits();
     addr_mask = ((1ull << phys_bits) - 1) & ~((1ull << 12) - 1);
 
     rdmsrl(MSR_MTRRcap, mtrr_cap);
diff --git a/xen/arch/x86/guest/hyperv/hyperv.c b/xen/arch/x86/guest/hyperv/hyperv.c
index 8d38313d7a..f986c1a805 100644
--- a/xen/arch/x86/guest/hyperv/hyperv.c
+++ b/xen/arch/x86/guest/hyperv/hyperv.c
@@ -18,17 +18,27 @@
  *
  * Copyright (c) 2019 Microsoft.
  */
+#include <xen/version.h>
 #include <xen/init.h>
 
+#include <asm/fixmap.h>
 #include <asm/guest.h>
 #include <asm/guest/hyperv-tlfs.h>
+#include <asm/processor.h>
 
 struct ms_hyperv_info __read_mostly ms_hyperv;
 
-static const struct hypervisor_ops ops = {
-    .name = "Hyper-V",
-};
+static uint64_t generate_guest_id(void)
+{
+    uint64_t id = 0;
+
+    id = (uint64_t)HV_XEN_VENDOR_ID << 48;
+    id |= (xen_major_version() << 16) | xen_minor_version();
+
+    return id;
+}
 
+static const struct hypervisor_ops ops;
 const struct hypervisor_ops *__init hyperv_probe(void)
 {
     uint32_t eax, ebx, ecx, edx;
@@ -72,6 +82,43 @@ const struct hypervisor_ops *__init hyperv_probe(void)
     return &ops;
 }
 
+static void __init setup_hypercall_page(void)
+{
+    union hv_x64_msr_hypercall_contents hypercall_msr;
+    union hv_guest_os_id guest_id;
+    unsigned long mfn;
+
+    rdmsrl(HV_X64_MSR_GUEST_OS_ID, guest_id.raw);
+    if ( !guest_id.raw )
+    {
+        guest_id.raw = generate_guest_id();
+        wrmsrl(HV_X64_MSR_GUEST_OS_ID, guest_id.raw);
+    }
+
+    rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
+    if ( !hypercall_msr.enable )
+    {
+        mfn = ((1ull << paddr_bits) - 1) >> HV_HYP_PAGE_SHIFT;
+        hypercall_msr.enable = 1;
+        hypercall_msr.guest_physical_address = mfn;
+        wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
+    } else {
+        mfn = hypercall_msr.guest_physical_address;
+    }
+
+    set_fixmap_x(FIX_X_HYPERV_HCALL, mfn << PAGE_SHIFT);
+}
+
+static void __init setup(void)
+{
+    setup_hypercall_page();
+}
+
+static const struct hypervisor_ops ops = {
+    .name = "Hyper-V",
+    .setup = setup,
+};
+
 /*
  * Local variables:
  * mode: C
diff --git a/xen/include/asm-x86/guest/hyperv-tlfs.h b/xen/include/asm-x86/guest/hyperv-tlfs.h
index 05c4044976..5d37efd2d2 100644
--- a/xen/include/asm-x86/guest/hyperv-tlfs.h
+++ b/xen/include/asm-x86/guest/hyperv-tlfs.h
@@ -318,15 +318,16 @@ struct ms_hyperv_tsc_page {
  *
  * Bit(s)
  * 63 - Indicates if the OS is Open Source or not; 1 is Open Source
- * 62:56 - Os Type; Linux is 0x100
+ * 62:56 - Os Type; Linux 0x100, FreeBSD 0x200, Xen 0x300
  * 55:48 - Distro specific identification
- * 47:16 - Linux kernel version number
+ * 47:16 - Guest OS version number
  * 15:0  - Distro specific identification
  *
  *
  */
 
 #define HV_LINUX_VENDOR_ID              0x8100
+#define HV_XEN_VENDOR_ID                0x8300
 union hv_guest_os_id
 {
     uint64_t raw;
