VTd/dmar: Tweak how the DMAR table is clobbered

Intead of clobbering DMAR -> XMAR and back, clobber to RMAD instead. This
means that changing the signature does not alter the checksum, which allows
the clobbering/unclobbering to be peformed atomically and idempotently, which
is an advantage on the kexec path which can reenter acpi_dmar_reinstate().

This DMAR clobbering was introduced by
83904107a3 which claims that the dom0 VT-d
driver was capable of playing with the IOMMU(s) while Xen was also using
them. An alternative approach might be to leave the DMAR table alone
and sprinkle some iomem_deny_access() around to forcibly prevent dom0
from playing but this is simpler.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix>
CC: Yang Zhang <yang.z.zhang@intel>
Acked-by: Kevin Tian <kevin.tian@intel>

xen/drivers/passthrough/vtd/dmar.c

@@ -28,6 +28,7 @@
 #include <xen/xmalloc.h>
 #include <xen/pci.h>
 #include <xen/pci_regs.h>
+#include <asm/atomic.h>
 #include <asm/string.h>
 #include "dmar.h"
 #include "iommu.h"
@@ -838,8 +839,7 @@ static int __init acpi_parse_dmar(struct acpi_table_header *table)
 
 out:
     /* Zap ACPI DMAR signature to prevent dom0 using vt-d HW. */
-    dmar->header.signature[0] = 'X';
-    dmar->header.checksum -= 'X'-'D';
+    acpi_dmar_zap();
     return ret;
 }
 
@@ -867,18 +867,18 @@ int __init acpi_dmar_init(void)
 
 void acpi_dmar_reinstate(void)
 {
-    if ( dmar_table == NULL )
-        return;
-    dmar_table->signature[0] = 'D';
-    dmar_table->checksum += 'X'-'D';
+    uint32_t sig = 0x52414d44; /* "DMAR" */
+
+    if ( dmar_table )
+        write_atomic((uint32_t*)&dmar_table->signature[0], sig);
 }
 
 void acpi_dmar_zap(void)
 {
-    if ( dmar_table == NULL )
-        return;
-    dmar_table->signature[0] = 'X';
-    dmar_table->checksum -= 'X'-'D';
+    uint32_t sig = 0x44414d52; /* "RMAD" - doesn't alter table checksum */
+
+    if ( dmar_table )
+        write_atomic((uint32_t*)&dmar_table->signature[0], sig);
 }
 
 int platform_supports_intremap(void)