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