On Sun, Aug 11, 2019 at 10:26:18AM +0800, piaojun wrote: > On 2019/8/9 16:21, Stefan Hajnoczi wrote: > > On Thu, Aug 08, 2019 at 10:53:16AM +0100, Dr. David Alan Gilbert wrote: > >> * Stefan Hajnoczi (stefa...@redhat.com) wrote: > >>> On Wed, Aug 07, 2019 at 04:57:15PM -0400, Vivek Goyal wrote: > >>> 3. Can READ/WRITE be performed directly in QEMU via a separate virtqueue > >>> to eliminate the bad address problem? > >> > >> Are you thinking of doing all read/writes that way, or just the corner > >> cases? It doesn't seem worth it for the corner cases unless you're > >> finding them cropping up in real work loads. > > > > Send all READ/WRITE requests to QEMU instead of virtiofsd. > > > > Only handle metadata requests in virtiofsd (OPEN, RELEASE, READDIR, > > MKDIR, etc). > > > > Sorry for not catching your point, and I like the virtiofsd to do > READ/WRITE requests and qemu handle metadata requests, as virtiofsd is > good at processing dataplane things due to thread-pool and CPU > affinity(maybe in the future). As you said, virtiofsd is just acting as > a vhost-user device which should care less about ctrl request. > > If our concern is improving mmap/write/read performance, why not adding > a delay worker for unmmap which could decrease the ummap times. Maybe > virtiofsd could still handle both data and meta requests by this way.
Doing READ/WRITE in QEMU solves the problem that vhost-user slaves only have access to guest RAM regions. If a guest transfers other memory, like an address in the DAX Window, to/from the vhost-user device then virtqueue buffer address translation fails. Dave added a code path that bounces such accesses through the QEMU process using the VHOST_USER_SLAVE_FS_IO slavefd request, but it would be simpler, faster, and cleaner to do I/O in QEMU in the first place. What I don't like about moving READ/WRITE into QEMU is that we need to use even more virtqueues for multiqueue operation :). Stefan
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