Re: [Virtio-fs] [PATCH 3/4] virtiofsd: use file-backend memory region for virtiofsd's cache area

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, May 17, 2019 at 07:28:22PM -0700, Liu Bo wrote:

[..]
> > > > > > That case happened with cache=none and the dax mount option.
> > > > > > 
> > > > > > The general problem is when FUSE_READ/FUSE_WRITE is sent and the buffer
> > > > > > is outside guest RAM.
> > > 
> > > Stefan,
> > > 
> > > Can this be emulated by sending a request to qemu? If virtiofsd can detect
> > > that source/destination of READ/WRITE is not guest RAM, can it forward
> > > message to qemu to do this operation (which has access to all the DAX
> > > windows)?
> > > 
> > > This probably will mean introducing new messages like
> > > setupmapping/removemapping messages between virtiofsd/qemu.  
> > 
> > Yes, interesting idea!
> > 
> > When virtiofsd is unable to map the virtqueue iovecs due to addresses
> > outside guest RAM, it could forward READ/WRITE requests to QEMU along
> > with the file descriptor.  It would be slow but fixes the problem.
> >
> 
> It is probably not easy to do.
> 
> Imagine the following case,
> // foo1 is on a dax virtiofs, foo2 is on a nondax virtiofs
> 
> p = mmap(foo1, ...);
> write(foo2, p, ...);
> 
> virtiofsd where foo2 is using needs to interpret gpa from virtiofs
> where foo1 exists along with fd being foo1, but a write fuse_req
> doesn't have foo1's fd.
> 
> And are you suggesting that qemu goes to read the data on gpa and
> returns via vhost-user message?  or let this virtiofsd (foo2) do mmap
> on foo1 directly?

Hi Liu Bo,

Not sure I understand the question. Assuming foo2 virtiofs instances
is mounted with cache=none, write(foo2), will trigger FUSE_WRITE
request to daemon with source address being in dax window of first
virtiofs instance. 

This request should be forwarded to qemu which can just read from
foo1 mmaped area in qemu address space and write to foo2. I am assuming
before FUSE_WRITE comes in, mapping for foo1 has already been
establied.

IOW, foo1 is already mmaped in qemu address space so there is no
need to pass fd of foo1 in FUSE_WRITE(foo2) request. Did I miss
something?

Vivek