I have no idea why we didn't do so from the beginning.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/acpi/boot.c
+++ b/xen/arch/x86/acpi/boot.c
@@ -481,22 +481,23 @@ static int __init acpi_parse_fadt(struct
 	if (fadt->header.revision >= FADT2_REVISION_ID) {
 		/* FADT rev. 2 */
 		if (fadt->xpm_timer_block.space_id ==
-		    ACPI_ADR_SPACE_SYSTEM_IO)
+		    ACPI_ADR_SPACE_SYSTEM_IO) {
 			pmtmr_ioport = fadt->xpm_timer_block.address;
-		/*
-		 * "X" fields are optional extensions to the original V1.0
-		 * fields, so we must selectively expand V1.0 fields if the
-		 * corresponding X field is zero.
-	 	 */
-		if (!pmtmr_ioport)
-			pmtmr_ioport = fadt->pm_timer_block;
-	} else {
-		/* FADT rev. 1 */
+			pmtmr_width = fadt->xpm_timer_block.bit_width;
+		}
+	}
+	/*
+	 * "X" fields are optional extensions to the original V1.0
+	 * fields, so we must selectively expand V1.0 fields if the
+	 * corresponding X field is zero.
+ 	 */
+	if (!pmtmr_ioport) {
 		pmtmr_ioport = fadt->pm_timer_block;
+		pmtmr_width = fadt->pm_timer_length == 4 ? 24 : 0;
 	}
 	if (pmtmr_ioport)
-		printk(KERN_INFO PREFIX "PM-Timer IO Port: %#x\n",
-		       pmtmr_ioport);
+		printk(KERN_INFO PREFIX "PM-Timer IO Port: %#x (%u bits)\n",
+		       pmtmr_ioport, pmtmr_width);
 #endif
 
 	acpi_smi_cmd       = fadt->smi_command;
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -403,6 +403,7 @@ static struct platform_timesource __init
  */
 
 u32 __read_mostly pmtmr_ioport;
+unsigned int __initdata pmtmr_width;
 
 /* ACPI PM timer ticks at 3.579545 MHz. */
 #define ACPI_PM_FREQUENCY 3579545
@@ -417,9 +418,15 @@ static s64 __init init_pmtimer(struct pl
     u64 start;
     u32 count, target, mask = 0xffffff;
 
-    if ( pmtmr_ioport == 0 )
+    if ( !pmtmr_ioport || !pmtmr_width )
         return 0;
 
+    if ( pmtmr_width == 32 )
+    {
+        pts->counter_bits = 32;
+        mask = 0xffffffff;
+    }
+
     count = inl(pmtmr_ioport) & mask;
     start = rdtsc_ordered();
     target = count + CALIBRATE_VALUE(ACPI_PM_FREQUENCY);
--- a/xen/include/asm-x86/acpi.h
+++ b/xen/include/asm-x86/acpi.h
@@ -146,6 +146,7 @@ extern u32 x86_acpiid_to_apicid[];
 #define INVALID_ACPIID		(-1U)
 
 extern u32 pmtmr_ioport;
+extern unsigned int pmtmr_width;
 
 int acpi_dmar_init(void);
 void acpi_mmcfg_init(void);