Re: [virtio-dev] [PATCH v8 2/2] virtio-fs: add DAX window

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Halil Pasic (pasic@linux.ibm.com) wrote:
> On Thu, 29 Aug 2019 14:52:06 +0100
> Stefan Hajnoczi <stefanha@redhat.com> wrote:
> 
> > Describe how shared memory region ID 0 is the DAX window and how
> > FUSE_SETUPMAPPING maps file ranges into the window.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> > The FUSE_SETUPMAPPING message is part of the virtio-fs Linux patches:
> > https://gitlab.com/virtio-fs/linux/blob/virtio-fs/include/uapi/linux/fuse.h
> > 
> > v8:
> >  * Make language about using both FUSE_READ/FUSE_WRITE and the DAX
> >    Window clearer [Cornelia]
> > v7:
> >  * Clarify that the DAX Window is optional and can be used together with
> >    FUSE_READ/FUSE_WRITE requests [Cornelia]
> > v6:
> >  * Document timing side-channel attacks [Michael]
> > ---
> >  virtio-fs.tex | 66 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 66 insertions(+)
> > 
> > diff --git a/virtio-fs.tex b/virtio-fs.tex
> > index 1ae17f8..158d066 100644
> > --- a/virtio-fs.tex
> > +++ b/virtio-fs.tex
> > @@ -179,6 +179,62 @@ \subsubsection{Device Operation: High Priority Queue}\label{sec:Device Types / F
> >  
> >  The driver MUST anticipate that request queues are processed concurrently with the hiprio queue.
> >  
> > +\subsubsection{Device Operation: DAX Window}\label{sec:Device Types / File System Device / Device Operation / Device Operation: DAX Window}
> > +
> > +FUSE\_READ and FUSE\_WRITE requests transfer file contents between the
> > +driver-provided buffer and the device.  In cases where data transfer is
> > +undesirable, the device can map file contents into the DAX window shared memory
> > +region.  The driver then accesses file contents directly in device-owned memory
> > +without a data transfer.
> > +
> > +The DAX Window is an alternative mechanism for accessing file contents.
> > +FUSE\_READ/FUSE\_WRITE requests and DAX Window accesses are possible at the
> > +same time.  Providing the DAX Window is optional for devices.  Using the DAX
> > +Window is optional for drivers.
> > +
> > +Shared memory region ID 0 is called the DAX window.  Drivers map this shared
> > +memory region with writeback caching as if it were regular RAM.  The contents
> > +of the DAX window are undefined unless a mapping exists for that range.
> 
> This last paragraph is a bit concerning form s390x perspective. In case
> of a PCI transport the shared memory region is a chunk of PCI memory (and
> must be contained within the declared bar, as mandated by commit
> 855ad7af2bd6).
> 
> The PCI architecture on s390x is at the moment such, that PCI memory
> *can't be accessed like regular RAM* but specialized instructions have
> to be used. I've tried to rise concern about this multiple times. Thus
> the virtio spec would contradict itself a little (at least on s390x).
> 
> Of course for virtual zPCI devices we can make this work. But including
> this paragraph in the VIRTIO specification would mean if one were to
> implement this in HW it would not work for s390.
> 
> I don't have a anything better to propose, so I intend to vote yes
> for this. I just wanted to make sure, we all are aware of the
> consequences.

Thanks.

Note this is just specifying the way virtiofs uses the existing
(accepted) shared memory region spec.  You can add a CCW transport of
that spec to make it appropriate for 390 if needed.

Dave

> Sorry for bringing this up again so late.
> 
> Adding Christian and Pierre as cc.

> Regards,
> Halil
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

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