x86/efi: Reserve SMBIOS table region when EFI booting

Some EFI firmware implementations may place the SMBIOS table in RAM
marked as BootServicesData, which Xen does not consider as reserved.
When dom0 tries to access the SMBIOS, the region is not contained in the
initial P2M and it crashes with a page fault. To fix this, reserve the
SMBIOS region.

Also, fix the memcmp checks for existence of the SMBIOS.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/dmi_scan.c

@@ -189,6 +189,8 @@ static inline bool_t __init dmi_checksum(const void __iomem *buf,
 
 static u32 __initdata efi_dmi_address;
 static u32 __initdata efi_dmi_size;
+static u32 __initdata efi_smbios_address;
+static u32 __initdata efi_smbios_size;
 static u64 __initdata efi_smbios3_address;
 static u32 __initdata efi_smbios3_size;
 
@@ -201,7 +203,7 @@ void __init dmi_efi_get_table(const void *smbios, const void *smbios3)
 	const struct smbios_eps *eps = smbios;
 	const struct smbios3_eps *eps3 = smbios3;
 
-	if (eps3 && memcmp(eps3->anchor, "_SM3_", 5) &&
+	if (eps3 && memcmp(eps3->anchor, "_SM3_", 5) == 0 &&
 	    eps3->length >= sizeof(*eps3) &&
 	    dmi_checksum(eps3, eps3->length)) {
 		efi_smbios3_address = eps3->address;
@@ -209,14 +211,18 @@ void __init dmi_efi_get_table(const void *smbios, const void *smbios3)
 		return;
 	}
 
-	if (eps && memcmp(eps->anchor, "_SM_", 4) &&
+	if (eps && memcmp(eps->anchor, "_SM_", 4) == 0 &&
 	    eps->length >= sizeof(*eps) &&
-	    dmi_checksum(eps, eps->length) &&
-	    memcmp(eps->dmi.anchor, "_DMI_", 5) == 0 &&
+	    dmi_checksum(eps, eps->length)) {
+		efi_smbios_address = (u32)(long)eps;
+		efi_smbios_size = eps->length;
+
+		if (memcmp(eps->dmi.anchor, "_DMI_", 5) == 0 &&
 		    dmi_checksum(&eps->dmi, sizeof(eps->dmi))) {
 			efi_dmi_address = eps->dmi.address;
 			efi_dmi_size = eps->dmi.size;
 		}
+	}
 }
 
 const char *__init dmi_get_table(paddr_t *base, u32 *len)
@@ -236,6 +242,12 @@ const char *__init dmi_get_table(paddr_t *base, u32 *len)
 			instance |= 2;
 			return "DMI";
 		}
+		if (efi_smbios_size && !(instance & 4)) {
+			*base = efi_smbios_address;
+			*len = efi_smbios_size;
+			instance |= 4;
+			return "SMBIOS";
+		}
 	} else {
 		char __iomem *p = maddr_to_virt(0xF0000), *q;
 		union {