[Xen-devel] [PATCH v4 24/34] xsplice: Add support for alternatives

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

From: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>

Add support for applying alternative sections within xsplice payload.
At payload load time, apply an alternative sections that are found.

Also we add an test-case exercising a rather useless alternative
(patching a NOP with a NOP) - but it does exercise the code-path.

Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

---
Cc: Keir Fraser <keir@xxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

v2: Make a new alternative function that does not ASSERT on IRQs and
    don't disable IRQs in the code when loading payload.
v3: Include test-case
v4: Include check for size of alternatives and that it is not a 0 size
    section.
---
 xen/arch/x86/Makefile                    |  2 +-
 xen/arch/x86/alternative.c               | 20 ++++++++++++--------
 xen/arch/x86/test/xen_hello_world_func.c |  3 +++
 xen/common/xsplice.c                     | 16 ++++++++++++++++
 xen/include/asm-x86/alternative.h        |  6 ++++++
 5 files changed, 38 insertions(+), 9 deletions(-)

diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
index a2e3017..8a100be 100644
--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -6,7 +6,7 @@ subdir-y += mm
 subdir-$(CONFIG_XENOPROF) += oprofile
 subdir-y += x86_64
 
-obj-bin-y += alternative.init.o
+obj-bin-y += alternative.o
 obj-y += apic.o
 obj-y += bitops.o
 obj-bin-y += bzimage.init.o
diff --git a/xen/arch/x86/alternative.c b/xen/arch/x86/alternative.c
index 26ad2b9..e423d3a 100644
--- a/xen/arch/x86/alternative.c
+++ b/xen/arch/x86/alternative.c
@@ -28,7 +28,7 @@
 extern struct alt_instr __alt_instructions[], __alt_instructions_end[];
 
 #ifdef K8_NOP1
-static const unsigned char k8nops[] __initconst = {
+static const unsigned char k8nops[] = {
     K8_NOP1,
     K8_NOP2,
     K8_NOP3,
@@ -52,7 +52,7 @@ static const unsigned char * const k8_nops[ASM_NOP_MAX+1] 
__initconstrel = {
 #endif
 
 #ifdef P6_NOP1
-static const unsigned char p6nops[] __initconst = {
+static const unsigned char p6nops[] = {
     P6_NOP1,
     P6_NOP2,
     P6_NOP3,
@@ -75,7 +75,7 @@ static const unsigned char * const p6_nops[ASM_NOP_MAX+1] 
__initconstrel = {
 };
 #endif
 
-static const unsigned char * const *ideal_nops __initdata = k8_nops;
+static const unsigned char * const *ideal_nops = k8_nops;
 
 static int __init mask_nmi_callback(const struct cpu_user_regs *regs, int cpu)
 {
@@ -100,7 +100,7 @@ static void __init arch_init_ideal_nops(void)
 }
 
 /* Use this to add nops to a buffer, then text_poke the whole buffer. */
-static void __init add_nops(void *insns, unsigned int len)
+static void add_nops(void *insns, unsigned int len)
 {
     while ( len > 0 )
     {
@@ -127,7 +127,7 @@ static void __init add_nops(void *insns, unsigned int len)
  *
  * This routine is called with local interrupt disabled.
  */
-static void *__init text_poke_early(void *addr, const void *opcode, size_t len)
+static void *text_poke_early(void *addr, const void *opcode, size_t len)
 {
     memcpy(addr, opcode, len);
     sync_core();
@@ -142,15 +142,13 @@ static void *__init text_poke_early(void *addr, const 
void *opcode, size_t len)
  * APs have less capabilities than the boot processor are not handled.
  * Tough. Make sure you disable such features by hand.
  */
-static void __init apply_alternatives(struct alt_instr *start, struct 
alt_instr *end)
+void apply_alternatives_nocheck(struct alt_instr *start, struct alt_instr *end)
 {
     struct alt_instr *a;
     u8 *instr, *replacement;
     u8 insnbuf[MAX_PATCH_LEN];
     unsigned long cr0 = read_cr0();
 
-    ASSERT(!local_irq_is_enabled());
-
     printk(KERN_INFO "alt table %p -> %p\n", start, end);
 
     /* Disable WP to allow application of alternatives to read-only pages. */
@@ -190,6 +188,12 @@ static void __init apply_alternatives(struct alt_instr 
*start, struct alt_instr
     write_cr0(cr0);
 }
 
+void apply_alternatives(struct alt_instr *start, struct alt_instr *end)
+{
+    ASSERT(!local_irq_is_enabled());
+    apply_alternatives_nocheck(start, end);
+}
+
 void __init alternative_instructions(void)
 {
     nmi_callback_t saved_nmi_callback;
diff --git a/xen/arch/x86/test/xen_hello_world_func.c 
b/xen/arch/x86/test/xen_hello_world_func.c
index 81380a6..2465ce9 100644
--- a/xen/arch/x86/test/xen_hello_world_func.c
+++ b/xen/arch/x86/test/xen_hello_world_func.c
@@ -5,10 +5,13 @@
 
 #include <xen/config.h>
 #include <xen/types.h>
+#include <asm/nops.h>
+#include <asm/alternative.h>
 
 /* Our replacement function for xen_extra_version. */
 const char *xen_hello_world(void)
 {
+    alternative(ASM_NOP1, ASM_NOP1, 1);
     return "Hello World";
 }
 
diff --git a/xen/common/xsplice.c b/xen/common/xsplice.c
index 834fda3..e5867bd 100644
--- a/xen/common/xsplice.c
+++ b/xen/common/xsplice.c
@@ -526,8 +526,24 @@ static int prepare_payload(struct payload *payload,
         region->ex_end = (struct exception_table_entry *)(sec->load_addr + 
sec->sec->sh_size);
 
         sort_exception_table(region->ex, region->ex_end);
+
         region->priv |= CHECKING_EXCEPTION;
     }
+    sec = xsplice_elf_sec_by_name(elf, ".altinstructions");
+    if ( sec )
+    {
+        if ( !sec->sec->sh_size ||
+             (sec->sec->sh_size % sizeof (struct alt_instr)) )
+        {
+            dprintk(XENLOG_DEBUG, "%s%s: Wrong size of .alt_instr (exp:%lu vs 
%lu)!\n",
+                    XSPLICE, elf->name, sizeof (struct alt_instr),
+                    sec->sec->sh_size);
+            return -EINVAL;
+        }
+        apply_alternatives_nocheck((struct alt_instr *)sec->load_addr,
+                                   (struct alt_instr *)(sec->load_addr +
+                                   sec->sec->sh_size));
+    }
 #endif
     return 0;
 }
diff --git a/xen/include/asm-x86/alternative.h 
b/xen/include/asm-x86/alternative.h
index 1056630..d50c0b5 100644
--- a/xen/include/asm-x86/alternative.h
+++ b/xen/include/asm-x86/alternative.h
@@ -23,6 +23,12 @@ struct alt_instr {
     u8  replacementlen;     /* length of new instruction, <= instrlen */
 };
 
+/*
+ * An variant to be used on code that can be patched without many checks.
+ */
+extern void apply_alternatives_nocheck(struct alt_instr *start,
+                                       struct alt_instr *end);
+extern void apply_alternatives(struct alt_instr *start, struct alt_instr *end);
 extern void alternative_instructions(void);
 
 #define OLDINSTR(oldinstr)      "661:\n\t" oldinstr "\n662:\n"
-- 
2.5.0


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