[Xen-devel] [PATCH] xen/arm: Register re-mapped Xen area as a temporary virtual region

[Wei Chen <Wei.Chen@xxxxxxx>]

While we're doing apply_alternatives, we will generate new instructions
if required. The new instructions need to update the Xen text section,
but Xen text section is read-only. So we re-map Xen to a new virtual
address to enable write access.

The targets of the new generated instructions are located in this
re-mapped Xen area. But we haven't register this area as a virtual
region, so the checking code determines the targets are not in the
Xen text section, the new instructions could not be generated.

In this patch, we register the re-mapped Xen area as a temporary
virtual region to make the new instructions can be generated
successfully.

Signed-off-by: Wei Chen <Wei.Chen@xxxxxxx>
---
 xen/arch/arm/alternative.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/alternative.c b/xen/arch/arm/alternative.c
index 1d10f51..96859fc 100644
--- a/xen/arch/arm/alternative.c
+++ b/xen/arch/arm/alternative.c
@@ -24,6 +24,7 @@
 #include <xen/vmap.h>
 #include <xen/smp.h>
 #include <xen/stop_machine.h>
+#include <xen/virtual_region.h>
 #include <asm/alternative.h>
 #include <asm/atomic.h>
 #include <asm/byteorder.h>
@@ -154,8 +155,12 @@ static int __apply_alternatives_multi_stop(void *unused)
         int ret;
         struct alt_region region;
         mfn_t xen_mfn = _mfn(virt_to_mfn(_start));
-        unsigned int xen_order = get_order_from_bytes(_end - _start);
+        unsigned int xen_size = _end - _start;
+        unsigned int xen_order = get_order_from_bytes(xen_size);
         void *xenmap;
+        struct virtual_region patch_region = {
+            .list = LIST_HEAD_INIT(patch_region.list),
+        };
 
         BUG_ON(patched);
 
@@ -169,6 +174,15 @@ static int __apply_alternatives_multi_stop(void *unused)
         BUG_ON(!xenmap);
 
         /*
+         * If we generate a new branch instruction, the target will be
+         * calculated in this re-mapped Xen region. So we have to register
+         * this re-mapped Xen region as a virtual region temporarily.
+         */
+        patch_region.start = xenmap;
+        patch_region.end = xenmap + xen_size;
+        register_virtual_region(&patch_region);
+
+        /*
          * Find the virtual address of the alternative region in the new
          * mapping.
          * alt_instr contains relative offset, so the function
@@ -182,6 +196,8 @@ static int __apply_alternatives_multi_stop(void *unused)
         /* The patching is not expected to fail during boot. */
         BUG_ON(ret != 0);
 
+        unregister_virtual_region(&patch_region);
+
         vunmap(xenmap);
 
         /* Barriers provided by the cache flushing */
-- 
2.7.4


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