From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55776) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1f3jWs-0003Pf-TY for qemu-devel@nongnu.org; Wed, 04 Apr 2018 10:36:52 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1f3jWo-00014M-0B for qemu-devel@nongnu.org; Wed, 04 Apr 2018 10:36:50 -0400 Received: from indium.canonical.com ([91.189.90.7]:33402) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1f3jWn-00013j-PN for qemu-devel@nongnu.org; Wed, 04 Apr 2018 10:36:45 -0400 Received: from loganberry.canonical.com ([91.189.90.37]) by indium.canonical.com with esmtp (Exim 4.86_2 #2 (Debian)) id 1f3jWl-0001iq-AF for ; Wed, 04 Apr 2018 14:36:43 +0000 Received: from loganberry.canonical.com (localhost [127.0.0.1]) by loganberry.canonical.com (Postfix) with ESMTP id 2A6362E80E2 for ; Wed, 4 Apr 2018 14:36:43 +0000 (UTC) MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Date: Wed, 04 Apr 2018 14:23:10 -0000 From: ChristianEhrhardt <1740219@bugs.launchpad.net> Reply-To: Bug 1740219 <1740219@bugs.launchpad.net> Sender: bounces@canonical.com References: <151435405424.26166.7757673860586517511.malonedeb@gac.canonical.com> Message-Id: <152285179025.8904.5938601612745159483.malone@soybean.canonical.com> Errors-To: bounces@canonical.com Subject: [Qemu-devel] [Bug 1740219] Re: static linux-user ARM emulation has several-second startup time List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org I wasn't sure if you'd usually take the interim step to "Fix Committed", thanks Peter. -- = You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/1740219 Title: static linux-user ARM emulation has several-second startup time Status in QEMU: Fix Committed Status in qemu package in Ubuntu: Triaged Bug description: static linux-user emulation has several-second startup time My problem: I'm a Parabola packager, and I'm updating our qemu-user-static package from 2.8 to 2.11. With my new statically-linked 2.11, running `qemu-arm /my/arm-chroot/bin/true` went from taking 0.006s to 3s! This does not happen with the normal dynamically linked 2.11, or the old static 2.8. What happens is it gets stuck in `linux-user/elfload.c:init_guest_space()`. What `init_guest_space` does is map 2 parts of the address space: `[base, base+guest_size]` and `[base+0xffff0000, base+0xffff0000+page_size]`; where it must find an acceptable `base`. Its strategy is to `mmap(NULL, guest_size, ...)` decide where the first range is, and then check if that +0xffff0000 is also available. If it isn't, then it starts trying `mmap(base, ...)` for the entire address space from low-address to high-address. "Normally," it finds an accaptable `base` within the first 2 tries. With a static 2.11, it's taking thousands of tries. ---- Now, from my understanding, there are 2 factors working together to cause that in static 2.11 but not the other builds: - 2.11 increased the default `guest_size` from 0xf7000000 to 0xffff0000 - PIE (and thus ASLR) is disabled for static builds For some reason that I don't understand, with the smaller `guest_size` the initial `mmap(NULL, guest_size, ...)` usually returns an acceptable address range; but larger `guest_size` makes it consistently return a block of memory that butts right up against another already mapped chunk of memory. This isn't just true on the older builds, it's true with the 2.11 builds if I use the `-R` flag to shrink the `guest_size` back down to 0xf7000000. That is with linux-hardened 4.13.13 on x86-64. So then, it it falls back to crawling the entire address space; so it tries base=3D0x00001000. With ASLR, that probably succeeds. But with ASLR being disabled on static builds, the text segment is at 0x60000000; which is does not leave room for the needed 0xffff1000-size block before it. So then it tries base=3D0x00002000. And so on, more than 6000 times until it finally gets to and passes the text segment; calling mmap more than 12000 times. ---- I'm not sure what the fix is. Perhaps try to mmap a continuous chunk of size 0xffff1000, then munmap it and then mmap the 2 chunks that we actually need. The disadvantage to that is that it does not support the sparse address space that the current algorithm supports for `guest_size < 0xffff0000`. If `guest_size < 0xffff0000` *and* the big mmap fails, then it could fall back to a sparse search; though I'm not sure the current algorithm is a good choice for it, as we see in this bug. Perhaps it should inspect /proc/self/maps to try to find a suitable range before ever calling mmap? To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/1740219/+subscriptions