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[Qemu-devel] [PATCH 3/5] exec: Do not use absolute address hints for cod
From: |
Richard Henderson |
Subject: |
[Qemu-devel] [PATCH 3/5] exec: Do not use absolute address hints for code_gen_buffer with -fpie |
Date: |
Tue, 16 Oct 2012 17:30:12 +1000 |
The hard-coded addresses inside alloc_code_gen_buffer only make sense
if we're building an executable that will actually run at the address
we've put into the linker scripts.
When we're building with -fpie, the executable will run at some
random location chosen by the kernel. We get better placement for
the code_gen_buffer if we allow the kernel to place the memory,
as it will tend to to place it near the executable, based on the
PROT_EXEC bit.
Since code_gen_prologue is always inside the executable, this effect
is easily seen at the end of most TB, with the exit_tb opcode, and
with any calls to helper functions.
Signed-off-by: Richard Henderson <address@hidden>
---
exec.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/exec.c b/exec.c
index 6c0b2d7..5e33a3d 100644
--- a/exec.c
+++ b/exec.c
@@ -578,7 +578,12 @@ static inline void *alloc_code_gen_buffer(void)
/* Constrain the position of the buffer based on the host cpu.
Note that these addresses are chosen in concert with the
addresses assigned in the relevant linker script file. */
-# if defined(__x86_64__) && defined(MAP_32BIT)
+# if defined(__PIE__) || defined(__PIC__)
+ /* Don't bother setting a preferred location if we're building
+ a position-independent executable. We're more likely to get
+ an address near the main executable if we let the kernel
+ choose the address. */
+# elif defined(__x86_64__) && defined(MAP_32BIT)
/* Force the memory down into low memory with the executable.
Leave the choice of exact location with the kernel. */
flags |= MAP_32BIT;
--
1.7.11.7
- [Qemu-devel] [PATCH v4 0/5] Better allocation of code_gen_buffer, Richard Henderson, 2012/10/16
- [Qemu-devel] [PATCH 1/5] exec: Split up and tidy code_gen_buffer, Richard Henderson, 2012/10/16
- [Qemu-devel] [PATCH 3/5] exec: Do not use absolute address hints for code_gen_buffer with -fpie,
Richard Henderson <=
- [Qemu-devel] [PATCH 4/5] exec: Allocate code_gen_prologue from code_gen_buffer, Richard Henderson, 2012/10/16
- [Qemu-devel] [PATCH 5/5] exec: Make MIN_CODE_GEN_BUFFER_SIZE private to exec.c, Richard Henderson, 2012/10/16
- [Qemu-devel] [PATCH 2/5] exec: Don't make DEFAULT_CODE_GEN_BUFFER_SIZE too large, Richard Henderson, 2012/10/16
- Re: [Qemu-devel] [PATCH v4 0/5] Better allocation of code_gen_buffer, Blue Swirl, 2012/10/20