Re: [PATCH 07/13] HV/Vmbus: Add SNP support for VMbus channel initiate message

On Wed, Jul 28, 2021 at 10:52:22AM -0400, Tianyu Lan wrote:

+	if (hv_is_isolation_supported()) {
+		vmbus_connection.monitor_pages_va[0]
+			= vmbus_connection.monitor_pages[0];
+		vmbus_connection.monitor_pages[0]
+			= memremap(msg->monitor_page1, HV_HYP_PAGE_SIZE,
+				   MEMREMAP_WB);
+		if (!vmbus_connection.monitor_pages[0])
+			return -ENOMEM;
+
+		vmbus_connection.monitor_pages_va[1]
+			= vmbus_connection.monitor_pages[1];
+		vmbus_connection.monitor_pages[1]
+			= memremap(msg->monitor_page2, HV_HYP_PAGE_SIZE,
+				   MEMREMAP_WB);
+		if (!vmbus_connection.monitor_pages[1]) {
+			memunmap(vmbus_connection.monitor_pages[0]);
+			return -ENOMEM;
+		}
+
+		memset(vmbus_connection.monitor_pages[0], 0x00,
+		       HV_HYP_PAGE_SIZE);
+		memset(vmbus_connection.monitor_pages[1], 0x00,
+		       HV_HYP_PAGE_SIZE);
+	}

Okay, let me see if I got this right. In Hyper-V Isolation VMs, when the
guest wants to make memory shared, it does":

	- Call to the Hypervisor the mark the pages shared. The
	  Hypervisor will do the RMP update and remap the pages at
	  (VTOM + pa)

	- The guest maps the memory again into its page-table, so that
	  the entries point to the correct GPA (which is above VTOM
	  now).

Or in other words, Hyper-V implements a hardware-independent and
configurable c-bit position, as the VTOM value is always power-of-two
aligned. Is that correct?
This would at least explain why there is no separate
allocation/dealloction of memory for the shared range.

Thanks,

	Joerg