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[Qemu-devel] Heads up: glibc preadv emulation breaks qemu on older kerne

From: Richard W.M. Jones
Subject: [Qemu-devel] Heads up: glibc preadv emulation breaks qemu on older kernels
Date: Thu, 11 Feb 2010 19:06:08 +0000
User-agent: Mutt/1.5.18 (2008-05-17)

This isn't a problem with qemu as such, but it may affect you if you
use qemu with a newer glibc on a Linux kernel which does not support
the preadv(2) syscall natively.  glibc will attempt to emulate
preadv(2) using pread(2) syscalls, but it doesn't align the user
buffer when it does this, and that can break things (if using
cache=off then the file is opened with O_DIRECT which has strict
alignment requirements).

The user visible effect of this bug is that blocks devices (of all
types) inside the guest throw large numbers of I/O errors.


One interesting thing is that qemu has its own preadv emulation (which
does the emulation correctly), but this is never used because qemu
never gets ENOSYS back from preadv.


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming blog: http://rwmj.wordpress.com
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