[PATCH] 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. It
leads to repeated mapping during Enlighten initialization in Linux.
Xen did not prevent guest from this, so it works but causes weird
wierd issues - one memory page, that was returned by memalign in U-boot
for Enlighten mapping was not returned to allocator and contained
shared info values during Linux run. Also Linux mapped it to another
place in memory again.

Now code, that restricts repeated mapping of Enlighten page, is about
to be merged to Xen:
https://lore.kernel.org/xen-devel/20220716145658.4175730-2-olekstysh@gmail.com/

So, without unmapping in U-boot, Linux will fail to start. As discussed
on the xen-devel mailing list, to fix this issue 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>
---
 drivers/xen/hypervisor.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/drivers/xen/hypervisor.c b/drivers/xen/hypervisor.c
index 2560894832..9ac377b618 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;
+	struct xen_memory_reservation reservation;
+	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)
+		BUG();
+
+	/*
+	 * After remove 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)
+		BUG();
+
+	/* 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