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Add a basic file system module for virtio-fs. This does not yet contain shared data support between host and guest or metadata coherency speedups. However it is already significantly faster than virtio-9p. Design Overview =============== With the goal of designing something with better performance and local file system semantics, a bunch of ideas were proposed. - Use fuse protocol (instead of 9p) for communication between guest and host. Guest kernel will be fuse client and a fuse server will run on host to serve the requests. - For data access inside guest, mmap portion of file in QEMU address space and guest accesses this memory using dax. That way guest page cache is bypassed and there is only one copy of data (on host). This will also enable mmap(MAP_SHARED) between guests. - For metadata coherency, there is a shared memory region which contains version number associated with metadata and any guest changing metadata updates version number and other guests refresh metadata on next access. This is yet to be implemented. How virtio-fs differs from existing approaches ============================================== The unique idea behind virtio-fs is to take advantage of the co-location of the virtual machine and hypervisor to avoid communication (vmexits). DAX allows file contents to be accessed without communication with the hypervisor. The shared memory region for metadata avoids communication in the common case where metadata is unchanged. By replacing expensive communication with cheaper shared memory accesses, we expect to achieve better performance than approaches based on network file system protocols. In addition, this also makes it easier to achieve local file system semantics (coherency). These techniques are not applicable to network file system protocols since the communications channel is bypassed by taking advantage of shared memory on a local machine. This is why we decided to build virtio-fs rather than focus on 9P or NFS. Caching Modes ============= Like virtio-9p, different caching modes are supported which determine the coherency level as well. The “cache=FOO” and “writeback” options control the level of coherence between the guest and host filesystems. - cache=none metadata, data and pathname lookup are not cached in guest. They are always fetched from host and any changes are immediately pushed to host. - cache=always metadata, data and pathname lookup are cached in guest and never expire. - cache=auto metadata and pathname lookup cache expires after a configured amount of time (default is 1 second). Data is cached while the file is open (close to open consistency). - writeback/no_writeback These options control the writeback strategy. If writeback is disabled, then normal writes will immediately be synchronized with the host fs. If writeback is enabled, then writes may be cached in the guest until the file is closed or an fsync(2) performed. This option has no effect on mmap-ed writes or writes going through the DAX mechanism. Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Vivek Goyal <vgoyal@redhat.com> Acked-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
51 lines
2.5 KiB
C
51 lines
2.5 KiB
C
#ifndef _LINUX_VIRTIO_IDS_H
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#define _LINUX_VIRTIO_IDS_H
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/*
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* Virtio IDs
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*
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* This header is BSD licensed so anyone can use the definitions to implement
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* compatible drivers/servers.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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* 3. Neither the name of IBM nor the names of its contributors
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* may be used to endorse or promote products derived from this software
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* without specific prior written permission.
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* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL IBM OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE. */
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#define VIRTIO_ID_NET 1 /* virtio net */
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#define VIRTIO_ID_BLOCK 2 /* virtio block */
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#define VIRTIO_ID_CONSOLE 3 /* virtio console */
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#define VIRTIO_ID_RNG 4 /* virtio rng */
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#define VIRTIO_ID_BALLOON 5 /* virtio balloon */
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#define VIRTIO_ID_RPMSG 7 /* virtio remote processor messaging */
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#define VIRTIO_ID_SCSI 8 /* virtio scsi */
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#define VIRTIO_ID_9P 9 /* 9p virtio console */
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#define VIRTIO_ID_RPROC_SERIAL 11 /* virtio remoteproc serial link */
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#define VIRTIO_ID_CAIF 12 /* Virtio caif */
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#define VIRTIO_ID_GPU 16 /* virtio GPU */
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#define VIRTIO_ID_INPUT 18 /* virtio input */
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#define VIRTIO_ID_VSOCK 19 /* virtio vsock transport */
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#define VIRTIO_ID_CRYPTO 20 /* virtio crypto */
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#define VIRTIO_ID_IOMMU 23 /* virtio IOMMU */
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#define VIRTIO_ID_FS 26 /* virtio filesystem */
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#define VIRTIO_ID_PMEM 27 /* virtio pmem */
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#endif /* _LINUX_VIRTIO_IDS_H */
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