From patchwork Fri Oct 12 21:20:49 2012 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Subject: [3/4] exec: Do not use absolute address hints for code_gen_buffer with -fpie Date: Fri, 12 Oct 2012 11:20:49 -0000 From: Richard Henderson X-Patchwork-Id: 191221 Message-Id: <1350076850-7099-4-git-send-email-rth@twiddle.net> To: qemu-devel@nongnu.org Cc: Blue Swirl 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 --- exec.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/exec.c b/exec.c index 386cc08..e5f8c30 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;