[Xen-devel] [PATCH 4/6] x86/boot: Clean up l?_bootmap[] construction

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The need for Xen to be identity mapped into the bootmap is not obvious, and
differs between the MB and EFI boot paths.  Furthermore, the EFI side is
further complicated by spraying non-identity aliases into the mix.

Simplify the EFI bootmap construction code to make exactly one identity-map of
Xen, which now matches the MB path.  Comment both pieces of logic, explaining
what the mappings are needed for.

Finally, leave a linker assert covering the fact that plenty of code blindly
assumes that Xen is less that 16M.  This wants fixing in due course.

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>

The MB path's dependency on Xen's identity mapping can be broken by having
trampoline_boot_cpu_entry switch the alias of gdt_48 it uses.

I took this approach first in an attempt to drop the bootmap entirely, but it
is incompatible with the EFI path, and would also work against other plans to
avoid using the trampoline during early boot.
---
 xen/arch/x86/boot/head.S    |  8 ++++++--
 xen/arch/x86/efi/efi-boot.h | 17 ++++++++++-------
 xen/arch/x86/xen.lds.S      |  3 +++
 3 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 7ee4511e26..f7d273ca36 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -668,7 +668,11 @@ trampoline_setup:
         add     %esi,sym_fs(__page_tables_start)-8(,%ecx,8)
 2:      loop    1b
 
-        /* Initialize L2 boot-map/direct map page table entries (16MB). */
+        /*
+         * Map Xen into the directmap (needed for early-boot pagetable
+         * handling/walking), and identity map Xen into bootmap (needed for
+         * the transition into long mode), using 2M superpages.
+         */
         lea     sym_esi(start),%ebx
         lea     
(1<<L2_PAGETABLE_SHIFT)*7+(PAGE_HYPERVISOR|_PAGE_PSE)(%ebx),%eax
         shr     $(L2_PAGETABLE_SHIFT-3),%ebx
@@ -678,7 +682,7 @@ trampoline_setup:
         sub     $(1<<L2_PAGETABLE_SHIFT),%eax
         loop    1b
 
-        /* Initialize L3 boot-map page directory entry. */
+        /* Initialize L3 boot-map page directory entries. */
         lea     
__PAGE_HYPERVISOR+(L2_PAGETABLE_ENTRIES*8)*3+sym_esi(l2_bootmap),%eax
         mov     $4,%ecx
 1:      mov     %eax,sym_fs(l3_bootmap)-8(,%ecx,8)
diff --git a/xen/arch/x86/efi/efi-boot.h b/xen/arch/x86/efi/efi-boot.h
index 676d616ff8..9c314e403a 100644
--- a/xen/arch/x86/efi/efi-boot.h
+++ b/xen/arch/x86/efi/efi-boot.h
@@ -584,21 +584,24 @@ static void __init efi_arch_memory_setup(void)
     if ( !efi_enabled(EFI_LOADER) )
         return;
 
-    /* Initialise L2 identity-map and boot-map page table entries (16MB). */
+    /*
+     * Map Xen into the directmap (NX, needed for early-boot pagetable
+     * handling/walking), and identity map Xen into bootmap (X, needed for the
+     * transition from the EFI pagetables to Xen), using 2M superpages.
+     */
     for ( i = 0; i < 8; ++i )
     {
         unsigned int slot = (xen_phys_start >> L2_PAGETABLE_SHIFT) + i;
         paddr_t addr = slot << L2_PAGETABLE_SHIFT;
 
         l2_identmap[slot] = l2e_from_paddr(addr, PAGE_HYPERVISOR|_PAGE_PSE);
-        slot &= L2_PAGETABLE_ENTRIES - 1;
         l2_bootmap[slot] = l2e_from_paddr(addr, __PAGE_HYPERVISOR|_PAGE_PSE);
     }
-    /* Initialise L3 boot-map page directory entries. */
-    l3_bootmap[l3_table_offset(xen_phys_start)] =
-        l3e_from_paddr((UINTN)l2_bootmap, __PAGE_HYPERVISOR);
-    l3_bootmap[l3_table_offset(xen_phys_start + (8 << L2_PAGETABLE_SHIFT) - 
1)] =
-        l3e_from_paddr((UINTN)l2_bootmap, __PAGE_HYPERVISOR);
+
+    /* Initialize L3 boot-map page directory entries. */
+    for ( i = 0; i < 4; ++i )
+        l3_bootmap[i] = l3e_from_paddr((UINTN)l2_bootmap + i * PAGE_SIZE,
+                                       __PAGE_HYPERVISOR);
 }
 
 static void __init efi_arch_handle_module(struct file *file, const CHAR16 
*name,
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index 111edb5360..7f82f64078 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -381,3 +381,6 @@ ASSERT((trampoline_end - trampoline_start) < 
TRAMPOLINE_SPACE - MBI_SPACE_MIN,
     "not enough room for trampoline and mbi data")
 ASSERT((wakeup_stack - wakeup_stack_start) >= WAKEUP_STACK_MIN,
     "wakeup stack too small")
+
+/* Plenty of boot code assumes that Xen isn't larger than 16M. */
+ASSERT(_end - _start <= MB(16), "Xen too large for early-boot assumptions")
-- 
2.11.0


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