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

• From: Cornelia Huck <cohuck@redhat.com>
• To: "Michael S. Tsirkin" <mst@redhat.com>
• Date: Wed, 9 Oct 2019 12:13:43 +0200

[a bit late to the party, sorry]

On Wed, 25 Sep 2019 06:38:53 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Tue, Sep 10, 2019 at 03:31:45PM +0100, Dr. David Alan Gilbert wrote:
> > * Halil Pasic (pasic@linux.ibm.com) wrote:
> > > On Tue, 10 Sep 2019 14:09:20 +0100
> > > "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> > >
> > > > * 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).

I saw a set of new instructions being introduced in the kernel which
seem to do just that, but I obviously don't know the details.

> > > > >
> > > > > 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.
> > > >
> > >
> > > On s390x we have both CCW and PCI transport. And that makes things even
> > > more complicated.
> > >
> > > IMHO specifying that virtiofs uses the existing shared memory
> > > specification like regular RAM conflicts with what is architecturally
> > > possible on s390x when the transport is PCI.
> >
> > OK.
> >
> >
> > > Because the fact that this is memory exposed by a PCI device and
> > > contained within a bar with the current s390 architecture implies that
> > > this memory can not be used as regular RAM but needs to be accessed via
> > > specialized instructions (PCI LOAD, PCI STORE). @Pierre: please confirm
> > > or disprove me.
> > >
> > > Of course both simply not doing DAX window on s390 if transport PCI,
> > > or conceptually extending the architecture (for virtual systems) and
> > > making it work in the non s390 way is an option.
> > >
> > > And yes for the CCW transport we can do whatever we want. And I think
> > > we do want regular RAM for CCW transport, because architecturally there
> > > is no way a CCW device can expose memory. So we would/will need to build
> > > something virtual. And if we do, we should do it the way it suits us
> > > best.
> >
> > Yes; I'm assuming you'd do whatever is appropriate on CCW and just not
> > use DAX with virtio-fs on a PCI transport.

That's a possibility; I'm not sure what the new pci instructions can do
for us.

Also, we can't do full DAX in Linux on s390 currently anyway; this
would need some work to enable device memory on s390 which is way
beyond the scope of this.

> >
> > Dave
>
> So the TC voted for the current proposal, accordingly I merged
> these patches. David, Stefan, Halil, would one of you be willing
> to add a patch to clarify the DAX behavior for such systems?

At the moment, I don't see a reason for an extra patch here. FWIW, we
have not even architectured shared regions for ccw yet.


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