Eric Blake
8a39b4d6e2
The raw format layer supports all flags via passthrough - but it only makes sense to pass through flags that the lower layer actually supports. The next patch gives stronger reasoning for why this is correct. At the moment, the raw format layer ignores the max_transfer limit of its protocol layer, and an attempt to do the qemu-io 'w -f 0 40m' to an NBD server that lacks FUA will pass the entire 40m request to the NBD driver, which then fragments the request itself into a 32m write, 8m write, and flush. But once the block layer starts honoring limits and fragmenting packets, the raw driver will hand the NBD driver two separate requests; if both requests have BDRV_REQ_FUA set, then this would result in a 32m write, flush, 8m write, and second flush. By having the raw layer no longer advertise FUA support when the protocol layer lacks it, we are back to a single flush at the block layer for the overall 40m request. Note that 'w -f -z 0 40m' does not currently exhibit the same problem, because there, the fragmentation does not occur until at the NBD layer (the raw layer has .bdrv_co_pwrite_zeroes, and the NBD layer doesn't advertise max_pwrite_zeroes to constrain things at the raw layer) - but the problem is latent and we would again have too many flushes without this patch once the NBD layer implements support for the new NBD_CMD_WRITE_ZEROES command, if it sets max_pwrite_zeroes to the same 32m limit as recommended by the NBD protocol. Signed-off-by: Eric Blake <eblake@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Message-id: 1468607524-19021-3-git-send-email-eblake@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
QEMU
QEMU is a generic and open source machine & userspace emulator and virtualizer.
QEMU is capable of emulating a complete machine in software without any need for hardware virtualization support. By using dynamic translation, it achieves very good performance. QEMU can also integrate with the Xen and KVM hypervisors to provide emulated hardware while allowing the hypervisor to manage the CPU. With hypervisor support, QEMU can achieve near native performance for CPUs. When QEMU emulates CPUs directly it is capable of running operating systems made for one machine (e.g. an ARMv7 board) on a different machine (e.g. an x86_64 PC board).
QEMU is also capable of providing userspace API virtualization for Linux and BSD kernel interfaces. This allows binaries compiled against one architecture ABI (e.g. the Linux PPC64 ABI) to be run on a host using a different architecture ABI (e.g. the Linux x86_64 ABI). This does not involve any hardware emulation, simply CPU and syscall emulation.
QEMU aims to fit into a variety of use cases. It can be invoked directly by users wishing to have full control over its behaviour and settings. It also aims to facilitate integration into higher level management layers, by providing a stable command line interface and monitor API. It is commonly invoked indirectly via the libvirt library when using open source applications such as oVirt, OpenStack and virt-manager.
QEMU as a whole is released under the GNU General Public License, version 2. For full licensing details, consult the LICENSE file.
Building
QEMU is multi-platform software intended to be buildable on all modern Linux platforms, OS-X, Win32 (via the Mingw64 toolchain) and a variety of other UNIX targets. The simple steps to build QEMU are:
mkdir build
cd build
../configure
make
Complete details of the process for building and configuring QEMU for all supported host platforms can be found in the qemu-tech.html file. Additional information can also be found online via the QEMU website:
http://qemu-project.org/Hosts/Linux
http://qemu-project.org/Hosts/W32
Submitting patches
The QEMU source code is maintained under the GIT version control system.
git clone git://git.qemu-project.org/qemu.git
When submitting patches, the preferred approach is to use 'git format-patch' and/or 'git send-email' to format & send the mail to the qemu-devel@nongnu.org mailing list. All patches submitted must contain a 'Signed-off-by' line from the author. Patches should follow the guidelines set out in the HACKING and CODING_STYLE files.
Additional information on submitting patches can be found online via the QEMU website
http://qemu-project.org/Contribute/SubmitAPatch
http://qemu-project.org/Contribute/TrivialPatches
Bug reporting
The QEMU project uses Launchpad as its primary upstream bug tracker. Bugs found when running code built from QEMU git or upstream released sources should be reported via:
https://bugs.launchpad.net/qemu/
If using QEMU via an operating system vendor pre-built binary package, it is preferable to report bugs to the vendor's own bug tracker first. If the bug is also known to affect latest upstream code, it can also be reported via launchpad.
For additional information on bug reporting consult:
http://qemu-project.org/Contribute/ReportABug
Contact
The QEMU community can be contacted in a number of ways, with the two main methods being email and IRC
- Mailing List: qemu-devel@nongnu.org
- Archives: http://lists.nongnu.org/mailman/listinfo/qemu-devel
- IRC: #qemu on irc.oftc.net
Information on additional methods of contacting the community can be found online via the QEMU website: