[Virtio-fs] Status of DAX for virtio-fs/virtiofsd?

["Alex Bennée" <alex.bennee@linaro.org>]

Hi,

I remember from the dim and distant past (probably a KVM Forum) that one
of the touted features of virtio-fs was the ability to get memory
efficiency savings by having a direct memory mapping the host pages into
the guest address space.

AFAICT the kernel side was merged a while ago, see 22f3787 (virtiofs:
set up virtio_fs dax_device) and related. However when investigating
what would be needed to support this for Xen guests using virtio-fs we
were confused as to what else was needed.

There were some patches for the old C daemon:

  Subject: [PATCH v3 00/26] virtiofs dax patches
  Date: Wed, 28 Apr 2021 12:00:34 +0100	[thread overview]
  Message-ID: <20210428110100.27757-1-dgilbert@redhat.com>

although they look like they were never merged and the C version of
virtiofsd has since been dropped from tools.

Looking at the supporting rust code (vhost_user/message.rs) there are a
number of additional messages:

    /// Virtio-fs draft: map file content into the window.
    FS_MAP = 6,
    /// Virtio-fs draft: unmap file content from the window.
    FS_UNMAP = 7,
    /// Virtio-fs draft: sync file content.
    FS_SYNC = 8,
    /// Virtio-fs draft: perform a read/write from an fd directly to GPA.
    FS_IO = 9,
    /// Upper bound of valid commands.
    MAX_CMD = 10,

that don't appear in the current canonical vhost-user reference in the
QEMU repository and the QEMU code certainly doesn't have implementations
for any of them. So I have some questions:

 * What VMM/daemon combinations has DAX been tested on?
 * Isn't it time the vhost-user spec is updated?
 * Is anyone picking up Dave's patches for the QEMU side of support?

Thanks,


-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro