Hi Len,

I have a patch here ported from the linux2.6.18-xen tree to make host S3 
suspend work under Xen (attached).

The salient part is this:

--- a/drivers/acpi/acpica/hwsleep.c
+++ b/drivers/acpi/acpica/hwsleep.c
@@ -46,6 +46,9 @@
 #include "accommon.h"
 #include "actables.h"
 
+#include <xen/acpi.h>
+#include <asm/xen/hypervisor.h>
+
 #define _COMPONENT          ACPI_HARDWARE
 ACPI_MODULE_NAME("hwsleep")
 
@@ -337,14 +340,19 @@ acpi_status asmlinkage acpi_enter_sleep_state(u8 sleep_state)
 
 	ACPI_FLUSH_CPU_CACHE();
 
-	status = acpi_hw_register_write(ACPI_REGISTER_PM1A_CONTROL,
-					PM1Acontrol);
-	if (ACPI_FAILURE(status)) {
-		return_ACPI_STATUS(status);
-	}
+	if (!xen_pv_domain()) {
+		status = acpi_hw_register_write(ACPI_REGISTER_PM1A_CONTROL,
+						PM1Acontrol);
+		if (ACPI_FAILURE(status)) {
+			return_ACPI_STATUS(status);
+		}
+
+		status = acpi_hw_register_write(ACPI_REGISTER_PM1B_CONTROL,
+						PM1Bcontrol);
+	} else
+		status = acpi_notify_hypervisor_state(sleep_state,
+						      PM1Acontrol, PM1Bcontrol);
 
-	status = acpi_hw_register_write(ACPI_REGISTER_PM1B_CONTROL,
-					PM1Bcontrol);
 	if (ACPI_FAILURE(status)) {
 		return_ACPI_STATUS(status);
 	}


where acpi_notify_hypervisor_state() more or less maps directly onto a 
Xen hypercall, which in turn performs the corresponding acpi operation 
from within Xen.

I'm guessing you won't find this patch appealing as-is because it sticks 
a great big if (xen) into an otherwise arch (and OS?) independent piece 
of code.  In this particular instance, its fairly easy to envisage 
encapsulating these two register writes into their own function which 
architectures can override, which makes it fairly easy for me to put a 
Xen hook in somewhere on the arch/x86 side of the fence.

But because Xen ends up doing the low-level cpu state save/restore, 
several other parts of the S3 suspend path can be skipped on the Linux 
side.  I'm wondering if you have any thoughts about how that can be 
done, or if putting the Xen code in as-is is acceptable?

(BTW, xen_pv_domain() expands to a constant 0 when Xen isn't enabled in 
the config, so the Xen-dependent bits will collapse down to nothing.  
But it is defined in asm/xen/hypervisor.h, which is only present on x86 
and ia64 architectures; on the other hand, believe those are the only 
architectures using acpi.)

Thanks,
    J