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[Xen-devel] [RFC v2 20/23] x86/pie: Add option to build the kernel as PIE for x86_64

Add the CONFIG_X86_PIE option which builds the kernel as a Position
Independent Executable (PIE). The kernel is currently build with the
mcmodel=kernel option which forces it to stay on the top 2G of the
virtual address space. With PIE, the kernel will be able to move below
the -2G limit increasing the KASLR range from 1GB to 3GB.

The modules do not support PIE due to how they are linked. Disable PIE
for them and default to mcmodel=kernel for now.

The PIE configuration is not yet compatible with XEN_PVH. Xen PVH
generates 32-bit assembly and uses a long jump to transition to 64-bit.
A long jump require an absolute reference that is not compatible with

Performance/Size impact:

Hackbench (50% and 1600% loads):
 - PIE disabled: no significant change (-0.50% / +0.50%)
 - PIE enabled: 7% to 8% on half load, 10% on heavy load.

These results are aligned with the different research on user-mode PIE
impact on cpu intensive benchmarks (around 10% on x86_64).

slab_test (average of 10 runs):
 - PIE disabled: no significant change (-1% / +1%)
 - PIE enabled: 3% to 4%

Kernbench (average of 10 Half and Optimal runs):
 Elapsed Time:
 - PIE disabled: no significant change (-0.22% / +0.06%)
 - PIE enabled: around 0.50%
 System Time:
 - PIE disabled: no significant change (-0.99% / -1.28%)
 - PIE enabled: 5% to 6%

Size of vmlinux (Ubuntu configuration):
 File size:
 - PIE disabled: 472928672 bytes (-0.000169% from baseline)
 - PIE enabled: 216878461 bytes (-54.14% from baseline)
 .text sections:
 - PIE disabled: 9373572 bytes (+0.04% from baseline)
 - PIE enabled: 9499138 bytes (+1.38% from baseline)

The big decrease in vmlinux file size is due to the lower number of
relocations appended to the file.

Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
 arch/x86/Kconfig  | 7 +++++++
 arch/x86/Makefile | 9 +++++++++
 2 files changed, 16 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 2632fa8e8945..a419f4110872 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2132,6 +2132,13 @@ config X86_GLOBAL_STACKPROTECTOR
        depends on CC_STACKPROTECTOR
+config X86_PIE
+       bool
+       depends on X86_64 && !XEN_PVH
+       select DEFAULT_HIDDEN
        bool "Support for hot-pluggable CPUs"
        depends on SMP
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 66af2704f096..05e01588b5af 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -45,8 +45,12 @@ export REALMODE_CFLAGS
 export BITS
+ifdef CONFIG_X86_PIE
+        LDFLAGS_vmlinux := -pie -shared -Bsymbolic
         LDFLAGS_vmlinux := --emit-relocs
 # Prevent GCC from generating any FP code by mistake.
@@ -141,7 +145,12 @@ else
         KBUILD_CFLAGS += $(cflags-y)
         KBUILD_CFLAGS += -mno-red-zone
+ifdef CONFIG_X86_PIE
+        KBUILD_CFLAGS += -fPIC
+        KBUILD_CFLAGS_MODULE += -fno-PIC -mcmodel=kernel
         KBUILD_CFLAGS += -mcmodel=kernel
         # -funit-at-a-time shrinks the kernel .text considerably
         # unfortunately it makes reading oopses harder.

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