[Xen-devel] [PATCH v3 2/7] xen/pvh: place the trampoline starting at MFN 1

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Since PVH guest jump straight into trampoline_setup trampoline_phys is
not initialized, thus the trampoline is relocated to address 0.

This works, but has the undesirable effect of having VA 0 mapped to
MFN 0, which means NULL pointed dereferences no longer trigger a page
fault.

In order to solve this, place the trampoline starting at MFN 1 and
reserve the memory used by it.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Wei Liu <wei.liu2@xxxxxxxxxx>
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
---
Changes since v2:
 - Clarify comments.

Changes since v1:
 - Expand comment in head.S.
 - Add a BUG_ON to check trampoline_phys value in the PVH case.
---
Should be backported to the 4.10.0-shim-comet branch.
---
 xen/arch/x86/boot/head.S | 3 +++
 xen/arch/x86/mm.c        | 9 +++++++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 4fe5a776b1..0f652cea11 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -411,6 +411,9 @@ __pvh_start:
         /* Skip bootloader setup and bios setup, go straight to trampoline */
         movb    $1, sym_esi(pvh_boot)
         movb    $1, sym_esi(skip_realmode)
+
+        /* Set trampoline_phys to use mfn 1 to avoid having a mapping at VA 0 
*/
+        movw    $0x1000, sym_esi(trampoline_phys)
         jmp     trampoline_setup
 
 #endif /* CONFIG_PVH_GUEST */
diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 1147a1afb1..275e68c417 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -292,9 +292,14 @@ void __init arch_init_memory(void)
     /*
      * First 1MB of RAM is historically marked as I/O.  If we booted PVH,
      * reclaim the space.  Irrespective, leave MFN 0 as special for the sake
-     * of 0 being a very common default value.
+     * of 0 being a very common default value. Also reserve the RAM needed by
+     * the trampoline on PVH starting at MFN 1.
      */
-    for ( i = 0; i < (pvh_boot ? 1 : 0x100); i++ )
+    BUG_ON(pvh_boot && trampoline_phys != 0x1000);
+    for ( i = 0;
+          i < (pvh_boot ? (1 + PFN_UP(trampoline_end - trampoline_start))
+                        : 0x100);
+          i++ )
         share_xen_page_with_guest(mfn_to_page(_mfn(i)),
                                   dom_io, XENSHARE_writable);
 
-- 
2.15.1


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