[PATCH v2] drivers: xen: unmap Enlighten page before jumping to Linux

[Dmytro Firsov]

This commit fixes issue with usage of Xen hypervisor shared info page.
Previously U-boot did not unmap it at the end of OS boot process. Xen
did not prevent guest from this. So, it worked, but caused wierd
issues - one memory page, that was returned by memalign in U-boot
for Enlighten mapping was not unmaped by Xen (shared_info values was
not removed from there) and returned to allocator. During the Linux
boot, it uses shared_info page as regular RAM page, which leads to
hypervisor shared info corruption.

So, to fix this issue, as discussed on the xen-devel mailing list, the
code should:
   1) Unmap the page
   2) Populate the area with memory using XENMEM_populate_physmap

This patch adds page unmapping via XENMEM_remove_from_physmap, fills
hole in address space where page was mapped via XENMEM_populate_physmap
and return this address to memory allocator for freeing.

Signed-off-by: Dmytro Firsov <dmytro_firsov at epam.com>
---

Changes in v2:
- Reword commit message to be more clear with purpose of the patch
- Change BUG() helper to panic() and add error messages
- Add struct zeroing during initialization
- Fix typo in comment
---
 drivers/xen/hypervisor.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/drivers/xen/hypervisor.c b/drivers/xen/hypervisor.c
index 2560894832..16c7c96c94 100644
--- a/drivers/xen/hypervisor.c
+++ b/drivers/xen/hypervisor.c
@@ -144,6 +144,36 @@ struct shared_info *map_shared_info(void *p)
 	return HYPERVISOR_shared_info;
 }
 
+void unmap_shared_info(void)
+{
+	xen_pfn_t shared_info_pfn = virt_to_pfn(HYPERVISOR_shared_info);
+	struct xen_remove_from_physmap xrfp = {0};
+	struct xen_memory_reservation reservation = {0};
+	xen_ulong_t nr_exts = 1;
+
+	xrfp.domid = DOMID_SELF;
+	xrfp.gpfn = shared_info_pfn;
+	if (HYPERVISOR_memory_op(XENMEM_remove_from_physmap, &xrfp) != 0)
+		panic("Failed to unmap HYPERVISOR_shared_info\n");
+
+	/*
+	 * After removing from physmap there will be a hole in address space on
+	 * HYPERVISOR_shared_info address, so to free memory allocated with
+	 * memalign and prevent exceptions during access to this page we need to
+	 * fill this 4KB hole with XENMEM_populate_physmap before jumping to Linux.
+	 */
+	reservation.domid = DOMID_SELF;
+	reservation.extent_order = 0;
+	reservation.address_bits = 0;
+	set_xen_guest_handle(reservation.extent_start, &shared_info_pfn);
+	reservation.nr_extents = nr_exts;
+	if (HYPERVISOR_memory_op(XENMEM_populate_physmap, &reservation) != nr_exts)
+		panic("Failed to populate memory on HYPERVISOR_shared_info addr\n");
+
+	/* Now we can return this to memory allocator */
+	free(HYPERVISOR_shared_info);
+}
+
 void do_hypervisor_callback(struct pt_regs *regs)
 {
 	unsigned long l1, l2, l1i, l2i;
@@ -251,4 +281,5 @@ void xen_fini(void)
 	fini_gnttab();
 	fini_xenbus();
 	fini_events();
+	unmap_shared_info();
 }
-- 
2.25.1