[Xen-devel] [PATCH v6.5 09/26] x86: Support compiling with indirect branch thunks

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Use -mindirect-branch=thunk-extern/-mindirect-branch-register when available.
To begin with, use the retpoline thunk.  Later work will add alternative
thunks which can be selected at boot time.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
v4:
 * New
---
 xen/arch/x86/Makefile         |  1 +
 xen/arch/x86/Rules.mk         |  7 +++++++
 xen/arch/x86/indirect_thunk.S | 28 ++++++++++++++++++++++++++++
 xen/arch/x86/xen.lds.S        |  1 +
 4 files changed, 37 insertions(+)
 create mode 100644 xen/arch/x86/indirect_thunk.S

diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
index d5d58a2..433233c 100644
--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -36,6 +36,7 @@ obj-y += io_apic.o
 obj-$(CONFIG_LIVEPATCH) += alternative.o livepatch.o
 obj-y += msi.o
 obj-y += msr.o
+obj-$(CONFIG_INDIRECT_THUNK) += indirect_thunk.o
 obj-y += ioport_emulate.o
 obj-y += irq.o
 obj-$(CONFIG_KEXEC) += machine_kexec.o
diff --git a/xen/arch/x86/Rules.mk b/xen/arch/x86/Rules.mk
index 568657e..abcc4d4 100644
--- a/xen/arch/x86/Rules.mk
+++ b/xen/arch/x86/Rules.mk
@@ -30,3 +30,10 @@ CFLAGS += -fno-asynchronous-unwind-tables
 ifneq ($(call cc-option,$(CC),-fvisibility=hidden,n),n)
 CFLAGS += -DGCC_HAS_VISIBILITY_ATTRIBUTE
 endif
+
+# Compile with thunk-extern, indirect-branch-register if avaiable.
+ifneq ($(call cc-option,$(CC),-mindirect-branch-register,n),n)
+CFLAGS += -mindirect-branch=thunk-extern -mindirect-branch-register
+CFLAGS += -DCONFIG_INDIRECT_THUNK
+export CONFIG_INDIRECT_THUNK=y
+endif
diff --git a/xen/arch/x86/indirect_thunk.S b/xen/arch/x86/indirect_thunk.S
new file mode 100644
index 0000000..4fef1c8
--- /dev/null
+++ b/xen/arch/x86/indirect_thunk.S
@@ -0,0 +1,28 @@
+#include <asm/asm_defns.h>
+
+.macro IND_THUNK_RETPOLINE reg:req
+        call 2f
+1:
+        lfence
+        jmp 1b
+2:
+        mov \reg, (%rsp)
+        ret
+.endm
+
+/*
+ * Build the __x86.indirect_thunk.* symbols.  Execution lands on an
+ * alternative patch point which implements one of the above THUNK_*'s
+ */
+.macro GEN_INDIRECT_THUNK name:req reg:req
+        .section .text.__x86.indirect_thunk.\name, "ax", @progbits
+
+ENTRY(__x86.indirect_thunk.\name)
+        IND_THUNK_RETPOLINE \reg
+.endm
+
+/* Instantiate GEN_INDIRECT_THUNK for each register except %rsp. */
+.irp enc, rax, rbx, rcx, rdx, rsi, rdi, rbp, \
+          r8, r9, r10, r11, r12, r13, r14, r15
+        GEN_INDIRECT_THUNK name=\enc, reg=%\enc
+.endr
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index d5e8821..345946f 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -59,6 +59,7 @@ SECTIONS
   .text : {
         _stext = .;            /* Text and read-only data */
        *(.text)
+       *(.text.__x86.*)
        *(.text.cold)
        *(.text.unlikely)
        *(.fixup)
-- 
2.1.4


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