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Date: Fri, 21 May 2010 15:28:17 +0200
From: "Edgar E. Iglesias" <edgar.iglesias@gmail.com>
Message-ID: <20100521132817.GA8021@edde.se.axis.com>
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Subject: [Qemu-devel] linux-user mmap bug
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To: qemu-devel@nongnu.org

Hi

I ran into an mmap problem linux-user emulating CRIS (32bit) on x86_64 hosts.
Guest asks for a non fixed mmap, QEMU tries the mmap but the kernel returns a
high 64bit address. QEMU notices that it wont fit in the guests 32bit ptr size
and retries with a low address but doesn't set the MAP_FIXED flag.

Was something like the following patch the intended behaviour or did I
missunderstand something? (it fixes my problem at least...)

Cheers

commit 96fd8e3fdedb697ba249f32245751a28979c3fab
Author: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Date:   Fri May 21 15:22:11 2010 +0200

    linux-user: Set MAP_FIXED for mmap address fixups.
    
    Signed-off-by: Edgar E. Iglesias <edgar@axis.com>

diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index 6a1d933..5308fe1 100644
--- a/linux-user/mmap.c
+++ b/linux-user/mmap.c
@@ -304,7 +304,11 @@ abi_ulong mmap_find_vma(abi_ulong start, abi_ulong size)
          *  - shmat() with SHM_REMAP flag
          */
         ptr = mmap(g2h(addr), size, PROT_NONE,
-                   MAP_ANONYMOUS|MAP_PRIVATE|MAP_NORESERVE, -1, 0);
+                   /* When the kernel returns addresses that the guest
+                      cannot use we might need to fallback to fixed
+                      allocations.  */
+                   (addr ? MAP_FIXED : 0)
+                   | MAP_ANONYMOUS|MAP_PRIVATE|MAP_NORESERVE, -1, 0);
 
         /* ENOMEM, if host address space has no memory */
         if (ptr == MAP_FAILED) {