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Re: [Qemu-devel] [PATCH 3/5] exec: Do not use absolute address hints for
From: |
Stefan Weil |
Subject: |
Re: [Qemu-devel] [PATCH 3/5] exec: Do not use absolute address hints for code_gen_buffer with -fpie |
Date: |
Sun, 18 Nov 2012 21:48:11 +0100 |
User-agent: |
Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.16) Gecko/20121027 Iceowl/1.0b1 Icedove/3.0.11 |
Am 16.10.2012 09:30, schrieb Richard Henderson:
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;
This patch breaks the TCG interpreter. Here is a test run on Debian
x86_64 (output shortened):
$ ./configure --enable-debug --enable-tcg-interpreter
--target-list=i386-softmmu --disable-docs
$ make
$ gdb --args i386-softmmu/qemu-system-i386 -L pc-bios
(gdb) r
Starting program: i386-softmmu/qemu-system-i386 -L pc-bios
[Thread debugging using libthread_db enabled]
[New Thread 0x7fffe8f73700 (LWP 1446)]
[New Thread 0x7fffe0470700 (LWP 1447)]
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffe8f73700 (LWP 1446)]
0x00005555558e7ded in tcg_qemu_tb_exec (cpustate=0x55555656f7e0,
tb_ptr=0xeab74acb <Address 0xeab74acb out of bounds>) at tci.c:445
445 TCGOpcode opc = tb_ptr[0];
(gdb) q
QEMU crashes early while executing a jmp opcode.
This patch restores functionality:
diff --git a/exec.c b/exec.c
index 8435de0..44e4504 100644
--- a/exec.c
+++ b/exec.c
@@ -564,7 +564,7 @@ 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(__PIE__) || defined(__PIC__)
+# if !defined(CONFIG_TCG_INTERPRETER) && (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
Regards
Stefan W.
- Re: [Qemu-devel] [PATCH 3/5] exec: Do not use absolute address hints for code_gen_buffer with -fpie,
Stefan Weil <=