[PATCH v3 4/5] x86: Port the basic alternative mechanism from Linux to Xen

[Feng Wu <feng.wu@intel.com>]

This patch ports the basic alternative mechanism from Linux to Xen.
With this mechanism, we can patch code based on the CPU features.

Signed-off-by: Feng Wu <feng.wu@intel.com>
---
 xen/arch/x86/Makefile             |   1 +
 xen/arch/x86/alternative.c        | 213 ++++++++++++++++++++++++++++++++++++++
 xen/arch/x86/setup.c              |   3 +
 xen/arch/x86/xen.lds.S            |  15 +++
 xen/include/asm-x86/alternative.h |  78 ++++++++++++++
 5 files changed, 310 insertions(+)
 create mode 100644 xen/arch/x86/alternative.c
 create mode 100644 xen/include/asm-x86/alternative.h

diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
index d502bdf..3734884 100644
--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -58,6 +58,7 @@ obj-y += crash.o
 obj-y += tboot.o
 obj-y += hpet.o
 obj-y += xstate.o
+obj-y += alternative.o
 
 obj-$(crash_debug) += gdbstub.o
 
diff --git a/xen/arch/x86/alternative.c b/xen/arch/x86/alternative.c
new file mode 100644
index 0000000..18cf17c
--- /dev/null
+++ b/xen/arch/x86/alternative.c
@@ -0,0 +1,213 @@
+/******************************************************************************
+ * alternative.c
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ */
+
+#include <xen/types.h>
+#include <asm/processor.h>
+#include <asm/alternative.h>
+#include <xen/init.h>
+#include <asm/system.h>
+#include <asm/traps.h>
+#include <asm/nmi.h>
+
+#define MAX_PATCH_LEN (255-1)
+
+extern struct alt_instr __alt_instructions[], __alt_instructions_end[];
+
+#ifdef K8_NOP1
+static const unsigned char k8nops[] __initconst = {
+    K8_NOP1,
+    K8_NOP2,
+    K8_NOP3,
+    K8_NOP4,
+    K8_NOP5,
+    K8_NOP6,
+    K8_NOP7,
+    K8_NOP8
+};
+static const unsigned char * const k8_nops[ASM_NOP_MAX+1] __initconst = {
+    NULL,
+    k8nops,
+    k8nops + 1,
+    k8nops + 1 + 2,
+    k8nops + 1 + 2 + 3,
+    k8nops + 1 + 2 + 3 + 4,
+    k8nops + 1 + 2 + 3 + 4 + 5,
+    k8nops + 1 + 2 + 3 + 4 + 5 + 6,
+    k8nops + 1 + 2 + 3 + 4 + 5 + 6 + 7
+};
+#endif
+
+#ifdef P6_NOP1
+static const unsigned char p6nops[] __initconst = {
+    P6_NOP1,
+    P6_NOP2,
+    P6_NOP3,
+    P6_NOP4,
+    P6_NOP5,
+    P6_NOP6,
+    P6_NOP7,
+    P6_NOP8
+};
+static const unsigned char * const p6_nops[ASM_NOP_MAX+1] __initconst = {
+    NULL,
+    p6nops,
+    p6nops + 1,
+    p6nops + 1 + 2,
+    p6nops + 1 + 2 + 3,
+    p6nops + 1 + 2 + 3 + 4,
+    p6nops + 1 + 2 + 3 + 4 + 5,
+    p6nops + 1 + 2 + 3 + 4 + 5 + 6,
+    p6nops + 1 + 2 + 3 + 4 + 5 + 6 + 7
+};
+#endif
+
+static const unsigned char * const *ideal_nops __initdata = k8_nops;
+
+static int __init mask_nmi_callback(struct cpu_user_regs *regs, int cpu)
+{
+    return 1;
+}
+
+static void __init arch_init_ideal_nops(void)
+{
+    /*
+     * Due to a decoder implementation quirk, some
+     * specific Intel CPUs actually perform better with
+     * the "k8_nops" than with the SDM-recommended NOPs.
+     */
+    if ( (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL) &&
+         !(boot_cpu_data.x86 == 6 &&
+           boot_cpu_data.x86_model >= 0x0f &&
+           boot_cpu_data.x86_model != 0x1c &&
+           boot_cpu_data.x86_model != 0x26 &&
+           boot_cpu_data.x86_model != 0x27 &&
+           boot_cpu_data.x86_model < 0x30) )
+        ideal_nops = p6_nops;
+}
+
+/* Use this to add nops to a buffer, then text_poke the whole buffer. */
+static void __init add_nops(void *insns, unsigned int len)
+{
+    while ( len > 0 )
+    {
+        unsigned int noplen = len;
+        if ( noplen > ASM_NOP_MAX )
+            noplen = ASM_NOP_MAX;
+        memcpy(insns, ideal_nops[noplen], noplen);
+        insns += noplen;
+        len -= noplen;
+    }
+}
+
+/*
+ * text_poke_early - Update instructions on a live kernel at boot time
+ * @addr: address to modify
+ * @opcode: source of the copy
+ * @len: length to copy
+ *
+ * When you use this code to patch more than one byte of an instruction
+ * you need to make sure that other CPUs cannot execute this code in parallel.
+ * Also no thread must be currently preempted in the middle of these
+ * instructions. And on the local CPU you need to be protected again NMI or MCE
+ * handlers seeing an inconsistent instruction while you patch.
+ *
+ * This routine is called with local interrupt disabled.
+ */
+static void *__init text_poke_early(void *addr, const void *opcode, size_t len)
+{
+    memcpy(addr, opcode, len);
+    sync_core();
+
+    return addr;
+}
+
+/*
+ * Replace instructions with better alternatives for this CPU type.
+ * This runs before SMP is initialized to avoid SMP problems with
+ * self modifying code. This implies that asymmetric systems where
+ * 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)
+{
+    struct alt_instr *a;
+    u8 *instr, *replacement;
+    u8 insnbuf[MAX_PATCH_LEN];
+
+    ASSERT(!local_irq_is_enabled());
+
+    printk(KERN_INFO "alt table %p -> %p\n", start, end);
+
+    /*
+     * The scan order should be from start to end. A later scanned
+     * alternative code can overwrite a previous scanned alternative code.
+     * Some kernel functions (e.g. memcpy, memset, etc) use this order to
+     * patch code.
+     *
+     * So be careful if you want to change the scan order to any other
+     * order.
+     */
+    for ( a = start; a < end; a++ )
+    {
+        instr = (u8 *)&a->instr_offset + a->instr_offset;
+        replacement = (u8 *)&a->repl_offset + a->repl_offset;
+        BUG_ON(a->replacementlen > a->instrlen);
+        BUG_ON(a->instrlen > sizeof(insnbuf));
+        BUG_ON(a->cpuid >= NCAPINTS * 32);
+        if ( !boot_cpu_has(a->cpuid) )
+            continue;
+
+        memcpy(insnbuf, replacement, a->replacementlen);
+
+        /* 0xe8/0xe9 is a relative jump; fix the offset. */
+        if ( (*insnbuf & 0xfe) == 0xe8 && a->replacementlen == 5 )
+            *(s32 *)(insnbuf + 1) += replacement - instr;
+
+        add_nops(insnbuf + a->replacementlen,
+                 a->instrlen - a->replacementlen);
+        text_poke_early(instr, insnbuf, a->instrlen);
+    }
+}
+
+void __init alternative_instructions(void)
+{
+    nmi_callback_t saved_nmi_callback;
+
+    arch_init_ideal_nops();
+
+    /*
+     * The patching is not fully atomic, so try to avoid local interruptions
+     * that might execute the to be patched code.
+     * Other CPUs are not running.
+     */
+    saved_nmi_callback = set_nmi_callback(mask_nmi_callback);
+
+    /*
+     * Don't stop machine check exceptions while patching.
+     * MCEs only happen when something got corrupted and in this
+     * case we must do something about the corruption.
+     * Ignoring it is worse than a unlikely patching race.
+     * Also machine checks tend to be broadcast and if one CPU
+     * goes into machine check the others follow quickly, so we don't
+     * expect a machine check to cause undue problems during to code
+     * patching.
+     */
+    apply_alternatives(__alt_instructions, __alt_instructions_end);
+
+    set_nmi_callback(saved_nmi_callback);
+}
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 508649d..d16453a 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -48,6 +48,7 @@
 #include <asm/setup.h>
 #include <xen/cpu.h>
 #include <asm/nmi.h>
+#include <asm/alternative.h>
 
 /* opt_nosmp: If true, secondary processors are ignored. */
 static bool_t __initdata opt_nosmp;
@@ -1288,6 +1289,8 @@ void __init noreturn __start_xen(unsigned long mbi_p)
     if ( cpu_has_fsgsbase )
         set_in_cr4(X86_CR4_FSGSBASE);
 
+    alternative_instructions();
+
     local_irq_enable();
 
     pt_pci_init();
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index 17db361..d4b1f1a 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -105,6 +105,12 @@ SECTIONS
   .init.text : {
        _sinittext = .;
        *(.init.text)
+       /*
+        * Here are the replacement instructions. The linker sticks them
+        * as binary blobs. The .altinstructions has enough data to get
+        * the address and the length of them to patch the kernel safely.
+        */
+       *(.altinstr_replacement)
        _einittext = .;
   } :text
   .init.data : {
@@ -120,6 +126,15 @@ SECTIONS
        __trampoline_seg_start = .;
        *(.trampoline_seg)
        __trampoline_seg_stop = .;
+       /*
+        * struct alt_inst entries. From the header (alternative.h):
+        * "Alternative instructions for different CPU types or capabilities"
+        * Think locking instructions on spinlocks.
+        */
+       . = ALIGN(8);
+        __alt_instructions = .;
+        *(.altinstructions)
+        __alt_instructions_end = .;
 
        . = ALIGN(8);
        __ctors_start = .;
diff --git a/xen/include/asm-x86/alternative.h b/xen/include/asm-x86/alternative.h
new file mode 100644
index 0000000..c746047
--- /dev/null
+++ b/xen/include/asm-x86/alternative.h
@@ -0,0 +1,78 @@
+#ifndef __X86_ALTERNATIVE_H__
+#define __X86_ALTERNATIVE_H__
+
+#include <asm/nops.h>
+
+#ifdef __ASSEMBLY__
+.macro altinstruction_entry orig alt feature orig_len alt_len
+        .long \orig - .
+        .long \alt - .
+        .word \feature
+        .byte \orig_len
+        .byte \alt_len
+.endm
+#else
+#include <xen/types.h>
+
+struct alt_instr {
+    s32 instr_offset;       /* original instruction */
+    s32 repl_offset;        /* offset to replacement instruction */
+    u16 cpuid;              /* cpuid bit set for replacement */
+    u8  instrlen;           /* length of original instruction */
+    u8  replacementlen;     /* length of new instruction, <= instrlen */
+};
+
+extern void alternative_instructions(void);
+
+#define OLDINSTR(oldinstr)      "661:\n\t" oldinstr "\n662:\n"
+
+#define b_replacement(number)   "663"#number
+#define e_replacement(number)   "664"#number
+
+#define alt_slen "662b-661b"
+#define alt_rlen(number) e_replacement(number)"f-"b_replacement(number)"f"
+
+#define ALTINSTR_ENTRY(feature, number)                                       \
+        " .long 661b - .\n"                             /* label           */ \
+        " .long " b_replacement(number)"f - .\n"        /* new instruction */ \
+        " .word " __stringify(feature) "\n"             /* feature bit     */ \
+        " .byte " alt_slen "\n"                         /* source len      */ \
+        " .byte " alt_rlen(number) "\n"                 /* replacement len */
+
+#define DISCARD_ENTRY(number)                           /* rlen <= slen */    \
+        " .byte 0xff + (" alt_rlen(number) ") - (" alt_slen ")\n"
+
+#define ALTINSTR_REPLACEMENT(newinstr, feature, number) /* replacement */     \
+        b_replacement(number)":\n\t" newinstr "\n" e_replacement(number) ":\n\t"
+
+/* alternative assembly primitive: */
+#define ALTERNATIVE(oldinstr, newinstr, feature)                        \
+        OLDINSTR(oldinstr)                                              \
+        ".pushsection .altinstructions,\"a\"\n"                         \
+        ALTINSTR_ENTRY(feature, 1)                                      \
+        ".popsection\n"                                                 \
+        ".pushsection .discard,\"aw\",@progbits\n"                      \
+        DISCARD_ENTRY(1)                                                \
+        ".popsection\n"                                                 \
+        ".pushsection .altinstr_replacement, \"ax\"\n"                  \
+        ALTINSTR_REPLACEMENT(newinstr, feature, 1)                      \
+        ".popsection"
+
+/*
+ * Alternative instructions for different CPU types or capabilities.
+ *
+ * This allows to use optimized instructions even on generic binary
+ * kernels.
+ *
+ * length of oldinstr must be longer or equal the length of newinstr
+ * It can be padded with nops as needed.
+ *
+ * For non barrier like inlines please define new variants
+ * without volatile and memory clobber.
+ */
+#define alternative(oldinstr, newinstr, feature)                        \
+        asm volatile (ALTERNATIVE(oldinstr, newinstr, feature) : : : "memory")
+
+#endif  /*  __ASSEMBLY__  */
+
+#endif /* __X86_ALTERNATIVE_H__ */
-- 
1.8.3.1