[Xen-devel] [PATCH v3 1/3] xen/x86: split boot trampoline into permanent and temporary part

[Juergen Gross <jgross@xxxxxxxx>]

The hypervisor needs a trampoline in low memory for early boot and
later for bringing up cpus and during wakeup from suspend. Today this
trampoline is kept completely even if most of it isn't needed later.

Split the trampoline into a permanent part and a temporary part needed
at early boot only. Introduce a new entry at the boundary.

Reduce the stack for wakeup code in order for the permanent
trampoline to fit in a single page. 4k of stack seems excessive, about
3k should be more than enough.

Add an ASSERT() to the linker script to ensure the wakeup stack is
always at least 3k.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
V3: - drop trampoline_boot_start label, use wakeup_stack instead
      (Jan Beulich)
V2: - don't reserve space for the wakeup stack, just use the rest of
      the permanent trampoline page (Jan Beulich)
    - add an build time assertion for wakeup stack size (Jan Beulich)
    - some minor fixes (Jan Beulich)
---
 xen/arch/x86/boot/trampoline.S | 32 ++++++++++++++++++++++++++++++--
 xen/arch/x86/boot/wakeup.S     |  5 ++---
 xen/arch/x86/xen.lds.S         |  2 ++
 xen/include/asm-x86/config.h   |  1 +
 4 files changed, 35 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/boot/trampoline.S b/xen/arch/x86/boot/trampoline.S
index 2715d17..a5d7b08 100644
--- a/xen/arch/x86/boot/trampoline.S
+++ b/xen/arch/x86/boot/trampoline.S
@@ -1,4 +1,20 @@
-        .code16
+/*
+ * Trampoline code relocated to low memory.
+ *
+ * Care must taken when referencing symbols: they live in the relocated
+ * trampoline and in the hypervisor binary. The hypervisor symbols can either
+ * be accessed by their virtual address or by the physical address. When
+ * using the physical address eventually the physical start address of the
+ * hypervisor must be taken into account: after early boot the hypervisor
+ * will copy itself to high memory and writes its physical start address to
+ * trampoline_xen_phys_start in the low memory trampoline copy.
+ *
+ * Parts of the trampoline are needed for early boot only, while some other
+ * parts are needed as long as the hypervisor is active (e.g. wakeup code
+ * after suspend, bringup code for secondary cpus). The permanent parts should
+ * not reference any temporary low memory trampoline parts as those parts are
+ * not guaranteed to persist.
+ */
 
 /* NB. bootsym() is only usable in real mode, or via BOOT_PSEUDORM_DS. */
 #undef bootsym
@@ -18,6 +34,10 @@
         .long 111b - (off) - .;            \
         .popsection
 
+/* Start of the permanent trampoline code. */
+
+        .code16
+
 GLOBAL(trampoline_realmode_entry)
         mov     %cs,%ax
         mov     %ax,%ds
@@ -131,6 +151,15 @@ start64:
         movabs  $__high_start,%rax
         jmpq    *%rax
 
+#include "wakeup.S"
+
+/* The first page of trampoline is permanent, the rest boot-time only. */
+/* Reuse the boot trampoline on the 1st trampoline page as stack for wakeup. */
+        .equ    wakeup_stack, trampoline_start + PAGE_SIZE
+        .global wakeup_stack
+
+/* From here on early boot only. */
+
         .code32
 trampoline_boot_cpu_entry:
         cmpb    $0,bootsym_rel(skip_realmode,5)
@@ -246,4 +275,3 @@ rm_idt: .word   256*4-1, 0, 0
 #include "mem.S"
 #include "edd.S"
 #include "video.S"
-#include "wakeup.S"
diff --git a/xen/arch/x86/boot/wakeup.S b/xen/arch/x86/boot/wakeup.S
index 08ea9b2..b901cb1 100644
--- a/xen/arch/x86/boot/wakeup.S
+++ b/xen/arch/x86/boot/wakeup.S
@@ -173,6 +173,5 @@ bogus_saved_magic:
         movw    $0x0e00 + 'S', 0xb8014
         jmp     bogus_saved_magic
 
-        .align  16
-        .fill   PAGE_SIZE,1,0
-wakeup_stack:
+/* Stack for wakeup: rest of first trampoline page. */
+ENTRY(wakeup_stack_start)
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index 2d0ee8e..1462194 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -335,3 +335,5 @@ ASSERT(IS_ALIGNED(__bss_end,        8), "__bss_end 
misaligned")
 
 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")
diff --git a/xen/include/asm-x86/config.h b/xen/include/asm-x86/config.h
index b9a6d94..d3ec2c3 100644
--- a/xen/include/asm-x86/config.h
+++ b/xen/include/asm-x86/config.h
@@ -75,6 +75,7 @@
 
 #define TRAMPOLINE_STACK_SPACE  PAGE_SIZE
 #define TRAMPOLINE_SPACE        (KB(64) - TRAMPOLINE_STACK_SPACE)
+#define WAKEUP_STACK_MIN        3072
 
 #define MBI_SPACE_MIN           (2 * PAGE_SIZE)
 
-- 
2.10.2


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