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71 lines
3.2 KiB
Markdown
71 lines
3.2 KiB
Markdown
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# How to use virtio-fs
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In the context of virtualization, it is always convenient to be able to share a directory from the host with the guest.
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__virtio-fs__, also known as __vhost-user-fs__ is a virtual device defined by the VIRTIO specification which allows any VMM to perform filesystem sharing.
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## Pre-requisites
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### The daemon
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This virtual device relies on the _vhost-user_ protocol, which assumes the backend (device emulation) is handled by a dedicated process running on the host. This daemon is called __virtiofsd__ and needs to be present on the host.
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_Install virtiofsd_
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```bash
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VIRTIOFSD_URL="$(curl --silent https://api.github.com/repos/intel/nemu/releases/latest | grep "browser_download_url" | grep "virtiofsd-x86_64" | grep -o 'https://.*[^ "]')"
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wget --quiet $VIRTIOFSD_URL -O "virtiofsd"
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chmod +x "virtiofsd"
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sudo setcap cap_sys_admin+epi "virtiofsd"
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```
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_Create shared directory_
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```bash
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mkdir /tmp/shared_dir
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```
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_Run virtiofsd_
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```bash
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./virtiofsd \
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-d \
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-o vhost_user_socket=/tmp/virtiofs \
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-o source=/tmp/shared_dir \
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-o cache=none
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```
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The `cache=none` option here is an important one as it tells the daemon not to try any memory mapping of the files, but instead to use the _virtqueues_ to convey the files content. The support for the memory mapping of the files will be added later.
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### The kernel
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In order to leverage __virtio-fs__ support from within the guest, and because the code has not been merged in upstream Linux kernel yet, it is required to build a custom kernel embedding the patches.
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The following branch `virtio-pmem_and_virtio-fs` on the repository https://github.com/sboeuf/linux.git includes all the needed patches to support __virtio-fs__.
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Make sure to build a kernel out of this branch that can be then used to boot the VM.
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## How to share directories with cloud-hypervisor
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### Start the VM
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Once the daemon is running, the option `--fs` from __cloud-hypervisor__ needs to be used.
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Direct kernel boot option is preferred since we need to provide the custom kernel including the __virtio-fs__ patches. We could boot from `hypervisor-fw` if we had previously edited the image to replace the kernel binary.
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Because _vhost-user_ expects a dedicated process (__virtiofsd__ in this case) to be able to access the guest RAM to communicate through the _virtqueues_ with the driver running in the guest, `--memory` option needs to be slightly modified. It needs to specify a backing file for the memory so that an external process can access it.
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Assuming you have `clear-kvm.img` and `custom-vmlinux.bin` on your system, here is the __cloud-hypervisor__ command you need to run:
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```bash
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./cloud-hypervisor \
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--cpus 4 \
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--memory "size=512,file=/dev/shm" \
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--disk clear-kvm.img \
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--kernel custom-vmlinux.bin \
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--cmdline "console=ttyS0 reboot=k panic=1 nomodules root=/dev/vda3" \
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--fs tag=virtiofs,sock=/tmp/virtiofs,num_queues=1,queue_size=512
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```
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### Mount the shared directory
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The last step is to mount the shared directory inside the guest, using the `virtio_fs` filesystem type.
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```bash
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mkdir mount_dir
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mount \
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-t virtio_fs /dev/null mount_dir/ \
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-o tag=virtiofs,rootmode=040000,user_id=0,group_id=0
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```
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The `tag` needs to be consistent with what has been provided through the __cloud-hypervisor__ command line.
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