From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:52058) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TMmfF-0004WE-S5 for qemu-devel@nongnu.org; Fri, 12 Oct 2012 17:21:04 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1TMmfE-0002fM-Nr for qemu-devel@nongnu.org; Fri, 12 Oct 2012 17:21:01 -0400 Received: from mail-pb0-f45.google.com ([209.85.160.45]:52942) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TMmfE-0002dX-ES for qemu-devel@nongnu.org; Fri, 12 Oct 2012 17:21:00 -0400 Received: by mail-pb0-f45.google.com with SMTP id rp2so3204247pbb.4 for ; Fri, 12 Oct 2012 14:21:00 -0700 (PDT) Sender: Richard Henderson From: Richard Henderson Date: Fri, 12 Oct 2012 14:20:49 -0700 Message-Id: <1350076850-7099-4-git-send-email-rth@twiddle.net> In-Reply-To: <1350076850-7099-1-git-send-email-rth@twiddle.net> References: <1350076850-7099-1-git-send-email-rth@twiddle.net> Subject: [Qemu-devel] [PATCH 3/4] exec: Do not use absolute address hints for code_gen_buffer with -fpie List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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; -- 1.7.11.7