[Xen-devel] [PATCH 3/4] x86/boot: Create the l2_xenmap[] mappings dynamically

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The build-time construction of l2_xenmap[] imposes an arbitrary limit of 16M
total, which is a limit looking to be lifted.

Move l2_xenmap[] into the bss, and adjust both the BIOS and EFI paths to fill
it in dynamically, based on the final linked size of Xen.  For current builds,
this reduces the number of .text/etc mappings from 7 to 4.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

In principle, the non-EFI case could be made to work by having a post-link
script fill in a suitable number of _PAGE_PRESENT entries in l2_xenmap[].
This doesn't work for the EFI case, because pagetable relocation is instead
triggered on the ad-hoc relocation table, which would require the
_PAGE_PRESENT references to be in place before the link takes place.
---
 xen/arch/x86/boot/head.S    | 14 ++++++++++++++
 xen/arch/x86/boot/x86_64.S  | 23 ++++++++---------------
 xen/arch/x86/efi/efi-boot.h | 14 ++++++++++++++
 xen/arch/x86/xen.lds.S      |  3 +++
 4 files changed, 39 insertions(+), 15 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index c5acbf56ae..94bed4a2d3 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -668,6 +668,20 @@ trampoline_setup:
         add     %esi,sym_fs(__page_tables_start)-8(,%ecx,8)
 2:      loop    1b
 
+        /* Map Xen into the higher mappings using 2M superpages. */
+        lea     _PAGE_PSE + PAGE_HYPERVISOR_RWX + sym_esi(_start), %eax
+        mov     $sym_offs(_start),   %ecx   /* %eax = PTE to write        */
+        mov     $sym_offs(_end - 1), %edx
+        shr     $L2_PAGETABLE_SHIFT, %ecx   /* %ecx = First slot to write */
+        shr     $L2_PAGETABLE_SHIFT, %edx   /* %edx = Final slot to write */
+
+1:      mov     %eax, sym_offs(l2_xenmap)(%esi, %ecx, 8)
+        add     $1, %ecx
+        add     $1 << L2_PAGETABLE_SHIFT, %eax
+
+        cmp     %edx, %ecx
+        jbe     1b
+
         /*
          * Map Xen into the directmap (needed for early-boot pagetable
          * handling/walking), and identity map Xen into bootmap (needed for
diff --git a/xen/arch/x86/boot/x86_64.S b/xen/arch/x86/boot/x86_64.S
index aabf561b23..e63bece460 100644
--- a/xen/arch/x86/boot/x86_64.S
+++ b/xen/arch/x86/boot/x86_64.S
@@ -43,6 +43,14 @@ multiboot_ptr:
 GLOBAL(stack_start)
         .quad   cpu0_stack + STACK_SIZE - CPUINFO_sizeof
 
+        .section .bss.page_aligned, "aw", @nobits
+        .align PAGE_SIZE, 0
+
+/* L2 mapping the Xen text/data/bss region.  Uses 1x 4k page. */
+GLOBAL(l2_xenmap)
+        .fill L2_PAGETABLE_ENTRIES, 8, 0
+        .size l2_xenmap, . - l2_xenmap
+
         .section .data.page_aligned, "aw", @progbits
         .align PAGE_SIZE, 0
 /*
@@ -80,21 +88,6 @@ GLOBAL(l2_directmap)
         .fill 4 * L2_PAGETABLE_ENTRIES - 1, 8, 0
         .size l2_directmap, . - l2_directmap
 
-/*
- * L2 mapping the 1GB Xen text/data/bss region.  At boot it maps 16MB from
- * __image_base__, and is modified when Xen relocates itself.  Uses 1x 4k
- * page.
- */
-GLOBAL(l2_xenmap)
-        .quad 0
-        idx = 1
-        .rept 7
-        .quad sym_offs(__image_base__) + (idx << L2_PAGETABLE_SHIFT) + 
(PAGE_HYPERVISOR_RWX | _PAGE_PSE)
-        idx = idx + 1
-        .endr
-        .fill L2_PAGETABLE_ENTRIES - 8, 8, 0
-        .size l2_xenmap, . - l2_xenmap
-
 /* L2 mapping the fixmap.  Uses 1x 4k page. */
 l2_fixmap:
         idx = 0
diff --git a/xen/arch/x86/efi/efi-boot.h b/xen/arch/x86/efi/efi-boot.h
index 50d1499867..e750db6f5c 100644
--- a/xen/arch/x86/efi/efi-boot.h
+++ b/xen/arch/x86/efi/efi-boot.h
@@ -585,6 +585,20 @@ static void __init efi_arch_memory_setup(void)
     if ( !efi_enabled(EFI_LOADER) )
         return;
 
+    /*
+     * Map Xen into the higher mappings, using 2M superpages.
+     *
+     * NB: We are currently in physical mode, so a RIP-relative relocation
+     * against _start/_end gets their position as placed by the bootloader,
+     * not as expected in the final build.  This has arbitrary 2M alignment,
+     * so subtract xen_phys_start to get the appropriate slots in l2_xenmap[].
+     */
+    for ( i =  l2_table_offset((UINTN)_start   - xen_phys_start);
+          i <= l2_table_offset((UINTN)_end - 1 - xen_phys_start); ++i )
+        l2_xenmap[i] =
+            l2e_from_paddr(xen_phys_start + (i << L2_PAGETABLE_SHIFT),
+                           PAGE_HYPERVISOR_RWX | _PAGE_PSE);
+
     /* Check that there is at least 4G of mapping space in l2_*map[] */
     BUILD_BUG_ON((sizeof(l2_bootmap)   / L2_PAGETABLE_ENTRIES) < 4);
     BUILD_BUG_ON((sizeof(l2_directmap) / L2_PAGETABLE_ENTRIES) < 4);
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index 7f82f64078..7c351b9df3 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -359,6 +359,9 @@ ASSERT(__image_base__ > XEN_VIRT_START |
 ASSERT(kexec_reloc_size - kexec_reloc <= PAGE_SIZE, "kexec_reloc is too large")
 #endif
 
+/* The Multiboot setup paths depend on this to simplify superpage PTE 
creation. */
+ASSERT(IS_ALIGNED(_start,            MB(2)), "_start misaligned")
+
 ASSERT(IS_ALIGNED(__2M_text_end,     SECTION_ALIGN), "__2M_text_end 
misaligned")
 ASSERT(IS_ALIGNED(__2M_rodata_start, SECTION_ALIGN), "__2M_rodata_start 
misaligned")
 ASSERT(IS_ALIGNED(__2M_rodata_end,   SECTION_ALIGN), "__2M_rodata_end 
misaligned")
-- 
2.11.0


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