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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH v2] x86/alt: Support for automatic padding calculations
The correct amount of padding in an origin patch site can be calculated
automatically, based on the relative lengths of the replacements.
This requires a bit of trickery to calculate correctly, especially in the
ALTENRATIVE_2 case where a branchless max() calculation in needed. The
calculation is further complicated because GAS's idea of true is -1 rather
than 1, which is why the extra negations are required.
Additionally, have apply_alternatives() attempt to optimise the padding nops.
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
v2: Fix build with Clang.
---
xen/arch/x86/Rules.mk | 4 +++
xen/arch/x86/alternative.c | 32 ++++++++++++++++---
xen/include/asm-x86/alternative-asm.h | 60 +++++++++++++++++++++++++++++------
xen/include/asm-x86/alternative.h | 46 +++++++++++++++++++++------
4 files changed, 120 insertions(+), 22 deletions(-)
diff --git a/xen/arch/x86/Rules.mk b/xen/arch/x86/Rules.mk
index 9897dea..e169d67 100644
--- a/xen/arch/x86/Rules.mk
+++ b/xen/arch/x86/Rules.mk
@@ -24,6 +24,10 @@ $(call as-option-add,CFLAGS,CC,".equ \"x\"$$(comma)1", \
-U__OBJECT_LABEL__ -DHAVE_GAS_QUOTED_SYM \
'-D__OBJECT_LABEL__=$(subst $(BASEDIR)/,,$(CURDIR))/$$@')
+# GCC's idea of true is -1. Clang's idea is 1
+$(call as-option-add,CFLAGS,CC,\
+ ".if ((1 > 0) < 0); .error \"\";.endif",,-DHAVE_AS_NEGATIVE_TRUE)
+
CFLAGS += -mno-red-zone -fpic -fno-asynchronous-unwind-tables
# Xen doesn't use SSE interally. If the compiler supports it, also skip the
diff --git a/xen/arch/x86/alternative.c b/xen/arch/x86/alternative.c
index 51ca53e..e24db84 100644
--- a/xen/arch/x86/alternative.c
+++ b/xen/arch/x86/alternative.c
@@ -180,13 +180,37 @@ void init_or_livepatch apply_alternatives(const struct
alt_instr *start,
uint8_t *orig = ALT_ORIG_PTR(a);
uint8_t *repl = ALT_REPL_PTR(a);
uint8_t buf[MAX_PATCH_LEN];
+ unsigned int total_len = a->orig_len + a->pad_len;
- BUG_ON(a->repl_len > a->orig_len);
- BUG_ON(a->orig_len > sizeof(buf));
+ BUG_ON(a->repl_len > total_len);
+ BUG_ON(total_len > sizeof(buf));
BUG_ON(a->cpuid >= NCAPINTS * 32);
+ /* No replacement to make, but try to optimise any padding. */
if ( !boot_cpu_has(a->cpuid) )
+ {
+ unsigned int i;
+
+ if ( a->pad_len <= 1 )
+ continue;
+
+ /* Search the padding area for any byte which isn't a nop. */
+ for ( i = a->orig_len; i < total_len; ++i )
+ if ( orig[i] != 0x90 )
+ break;
+
+ /*
+ * Only make any changes if all padding bytes are unoptimised
+ * nops. With multiple alternatives over the same origin site, we
+ * may have already made a replacement, or optimised the nops.
+ */
+ if ( i != total_len )
+ continue;
+
+ add_nops(buf, a->pad_len);
+ text_poke(orig + a->orig_len, buf, a->pad_len);
continue;
+ }
memcpy(buf, repl, a->repl_len);
@@ -194,8 +218,8 @@ void init_or_livepatch apply_alternatives(const struct
alt_instr *start,
if ( a->repl_len >= 5 && (*buf & 0xfe) == 0xe8 )
*(int32_t *)(buf + 1) += repl - orig;
- add_nops(buf + a->repl_len, a->orig_len - a->repl_len);
- text_poke(orig, buf, a->orig_len);
+ add_nops(buf + a->repl_len, total_len - a->repl_len);
+ text_poke(orig, buf, total_len);
}
}
diff --git a/xen/include/asm-x86/alternative-asm.h
b/xen/include/asm-x86/alternative-asm.h
index 150bd1a..25f79fe 100644
--- a/xen/include/asm-x86/alternative-asm.h
+++ b/xen/include/asm-x86/alternative-asm.h
@@ -9,30 +9,55 @@
* enough information for the alternatives patching code to patch an
* instruction. See apply_alternatives().
*/
-.macro altinstruction_entry orig repl feature orig_len repl_len
+.macro altinstruction_entry orig repl feature orig_len repl_len pad_len
.long \orig - .
.long \repl - .
.word \feature
.byte \orig_len
.byte \repl_len
+ .byte \pad_len
.endm
#define orig_len (.L\@_orig_e - .L\@_orig_s)
+#define pad_len (.L\@_orig_p - .L\@_orig_e)
+#define total_len (.L\@_orig_p - .L\@_orig_s)
#define repl_len(nr) (.L\@_repl_e\()nr - .L\@_repl_s\()nr)
#define decl_repl(insn, nr) .L\@_repl_s\()nr: insn; .L\@_repl_e\()nr:
+/* GCC's idea of true is -1, while Clang's idea is 1. */
+#ifdef HAVE_AS_NEGATIVE_TRUE
+# define as_true(x) (-(x))
+#else
+# define as_true(x) (x)
+#endif
+
+#define as_max(a, b) ((a) ^ (((a) ^ (b)) & -as_true((a) < (b))))
+
.macro ALTERNATIVE oldinstr, newinstr, feature
.L\@_orig_s:
\oldinstr
.L\@_orig_e:
+ /*
+ * Calculate the difference in size between the replacement and original
+ * instructions, to derive how much padding to introduce.
+ */
+ .L\@_diff = repl_len(1) - orig_len
+
+ .skip as_true(.L\@_diff > 0) * .L\@_diff, 0x90
+.L\@_orig_p:
.pushsection .altinstructions, "a", @progbits
altinstruction_entry .L\@_orig_s, .L\@_repl_s1, \feature, \
- orig_len, repl_len(1)
+ orig_len, repl_len(1), pad_len
.section .discard, "a", @progbits
- /* Assembler-time check that \newinstr isn't longer than \oldinstr. */
- .byte 0xff + repl_len(1) - orig_len
+ /*
+ * Assembler-time checks:
+ * - total_len <= 255
+ * - \newinstr <= total_len
+ */
+ .byte total_len
+ .byte 0xff + repl_len(1) - total_len
.section .altinstr_replacement, "ax", @progbits
@@ -45,18 +70,31 @@
.L\@_orig_s:
\oldinstr
.L\@_orig_e:
+ /*
+ * Calculate the difference in size between the largest replacement and
+ * the original instructions, to derive how much padding to introduce.
+ */
+ .L\@_diff = as_max(repl_len(1), repl_len(2)) - orig_len
+
+ .skip as_true(.L\@_diff > 0) * .L\@_diff, 0x90
+.L\@_orig_p:
.pushsection .altinstructions, "a", @progbits
altinstruction_entry .L\@_orig_s, .L\@_repl_s1, \feature1, \
- orig_len, repl_len(1)
+ orig_len, repl_len(1), pad_len
altinstruction_entry .L\@_orig_s, .L\@_repl_s2, \feature2, \
- orig_len, repl_len(2)
+ orig_len, repl_len(2), pad_len
.section .discard, "a", @progbits
- /* Assembler-time check that \newinstr{1,2} aren't longer than \oldinstr.
*/
- .byte 0xff + repl_len(1) - orig_len
- .byte 0xff + repl_len(2) - orig_len
+ /*
+ * Assembler-time checks:
+ * - total_len <= 255
+ * - \newinstr* <= total_len
+ */
+ .byte total_len
+ .byte 0xff + repl_len(1) - total_len
+ .byte 0xff + repl_len(2) - total_len
.section .altinstr_replacement, "ax", @progbits
@@ -66,8 +104,12 @@
.popsection
.endm
+#undef as_max
+#undef as_true
#undef decl_repl
#undef repl_len
+#undef total_len
+#undef pad_len
#undef orig_len
#endif /* __ASSEMBLY__ */
diff --git a/xen/include/asm-x86/alternative.h
b/xen/include/asm-x86/alternative.h
index bcad3ee..d53cea0 100644
--- a/xen/include/asm-x86/alternative.h
+++ b/xen/include/asm-x86/alternative.h
@@ -8,12 +8,13 @@
#include <xen/stringify.h>
#include <xen/types.h>
-struct alt_instr {
+struct __packed alt_instr {
int32_t orig_offset; /* original instruction */
int32_t repl_offset; /* offset to replacement instruction */
uint16_t cpuid; /* cpuid bit set for replacement */
uint8_t orig_len; /* length of original instruction */
- uint8_t repl_len; /* length of new instruction, <= instrlen */
+ uint8_t repl_len; /* length of new instruction */
+ uint8_t pad_len; /* length of build-time padding */
};
#define __ALT_PTR(a,f) ((uint8_t *)((void *)&(a)->f + (a)->f))
@@ -26,43 +27,70 @@ extern void apply_alternatives(const struct alt_instr
*start,
const struct alt_instr *end);
extern void alternative_instructions(void);
-#define OLDINSTR(oldinstr) ".LXEN%=_orig_s:\n\t" oldinstr "\n.LXEN%=_orig_e:\n"
-
#define alt_orig_len "(.LXEN%=_orig_e - .LXEN%=_orig_s)"
+#define alt_pad_len "(.LXEN%=_orig_p - .LXEN%=_orig_e)"
+#define alt_total_len "(.LXEN%=_orig_p - .LXEN%=_orig_s)"
#define alt_repl_s(num) ".LXEN%=_repl_s"#num
#define alt_repl_e(num) ".LXEN%=_repl_e"#num
#define alt_repl_len(num) "(" alt_repl_e(num) " - " alt_repl_s(num) ")"
+/* GCC's idea of true is -1, while Clang's idea is 1. */
+#ifdef HAVE_AS_NEGATIVE_TRUE
+# define AS_TRUE "-"
+#else
+# define AS_TRUE ""
+#endif
+
+#define as_max(a, b) "(("a") ^ ((("a") ^ ("b")) & -("AS_TRUE"(("a") <
("b")))))"
+
+#define OLDINSTR_1(oldinstr, n1) \
+ ".LXEN%=_orig_s:\n\t" oldinstr "\n .LXEN%=_orig_e:\n\t" \
+ ".LXEN%=_diff = "alt_repl_len(n1)"-"alt_orig_len"\n\t" \
+ ".skip "AS_TRUE"(.LXEN%=_diff > 0) * .LXEN%=_diff, 0x90\n\t" \
+ ".LXEN%=_orig_p:\n\t"
+
+#define ALT_PADDING_LEN(n1, n2) \
+ as_max((alt_repl_len(n1), alt_repl_len(n2))"-"alt_orig_len
+
+#define OLDINSTR_2(oldinstr, n1, n2) \
+ ".LXEN%=_orig_s:\n\t" oldinstr "\n .LXEN%=_orig_e:\n\t" \
+ ".LXEN%=_diff = "ALT_PADDING_LEN(n1, n2)"\n\t" \
+ ".skip "AS_TRUE"(.LXEN%=_diff > 0) * .LXEN%=_diff, 0x90\n\t" \
+ ".LXEN%=_orig_p:\n\t"
+
#define ALTINSTR_ENTRY(feature, num) \
" .long .LXEN%=_orig_s - .\n" /* label */ \
" .long " alt_repl_s(num)" - .\n" /* new instruction */ \
" .word " __stringify(feature) "\n" /* feature bit */ \
" .byte " alt_orig_len "\n" /* source len */ \
- " .byte " alt_repl_len(num) "\n" /* replacement len */
+ " .byte " alt_repl_len(num) "\n" /* replacement len */ \
+ " .byte " alt_pad_len "\n" /* padding len */
-#define DISCARD_ENTRY(num) /* repl <= orig */ \
- " .byte 0xff + (" alt_repl_len(num) ") - (" alt_orig_len ")\n"
+#define DISCARD_ENTRY(num) /* repl <= total */ \
+ " .byte 0xff + (" alt_repl_len(num) ") - (" alt_total_len ")\n"
#define ALTINSTR_REPLACEMENT(newinstr, num) /* replacement */ \
alt_repl_s(num)":\n\t" newinstr "\n" alt_repl_e(num) ":\n\t"
/* alternative assembly primitive: */
#define ALTERNATIVE(oldinstr, newinstr, feature) \
- OLDINSTR(oldinstr) \
+ OLDINSTR_1(oldinstr, 1) \
".pushsection .altinstructions, \"a\", @progbits\n" \
ALTINSTR_ENTRY(feature, 1) \
".section .discard, \"a\", @progbits\n" \
+ ".byte " alt_total_len "\n" /* total_len <= 255 */ \
DISCARD_ENTRY(1) \
".section .altinstr_replacement, \"ax\", @progbits\n" \
ALTINSTR_REPLACEMENT(newinstr, 1) \
".popsection\n"
#define ALTERNATIVE_2(oldinstr, newinstr1, feature1, newinstr2, feature2) \
- OLDINSTR(oldinstr) \
+ OLDINSTR_2(oldinstr, 1, 2) \
".pushsection .altinstructions, \"a\", @progbits\n" \
ALTINSTR_ENTRY(feature1, 1) \
ALTINSTR_ENTRY(feature2, 2) \
".section .discard, \"a\", @progbits\n" \
+ ".byte " alt_total_len "\n" /* total_len <= 255 */ \
DISCARD_ENTRY(1) \
DISCARD_ENTRY(2) \
".section .altinstr_replacement, \"ax\", @progbits\n" \
--
2.1.4
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