On Fri, 14 Dec 2018 13:44:34 +0000 Stefan Hajnoczi <stefa...@redhat.com> wrote:
> On Thu, Dec 13, 2018 at 01:38:23PM +0100, Cornelia Huck wrote: > > On Thu, 13 Dec 2018 13:24:31 +0100 > > David Hildenbrand <da...@redhat.com> wrote: > > > > > On 13.12.18 13:15, Dr. David Alan Gilbert wrote: > > > > * David Hildenbrand (da...@redhat.com) wrote: > > > >> On 13.12.18 11:00, Dr. David Alan Gilbert wrote: > > > >>> * David Hildenbrand (da...@redhat.com) wrote: > > > >>>> On 13.12.18 10:13, Dr. David Alan Gilbert wrote: > > > >>>>> * David Hildenbrand (da...@redhat.com) wrote: > > > >>>>>> On 10.12.18 18:12, Vivek Goyal wrote: > > > >>>>>>> Instead of assuming we had the fixed bar for the cache, use the > > > >>>>>>> value from the capabilities. > > > >>>>>>> > > > >>>>>>> Signed-off-by: Dr. David Alan Gilbert <dgilb...@redhat.com> > > > >>>>>>> --- > > > >>>>>>> fs/fuse/virtio_fs.c | 32 +++++++++++++++++--------------- > > > >>>>>>> 1 file changed, 17 insertions(+), 15 deletions(-) > > > >>>>>>> > > > >>>>>>> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c > > > >>>>>>> index 60d496c16841..55bac1465536 100644 > > > >>>>>>> --- a/fs/fuse/virtio_fs.c > > > >>>>>>> +++ b/fs/fuse/virtio_fs.c > > > >>>>>>> @@ -14,11 +14,6 @@ > > > >>>>>>> #include <uapi/linux/virtio_pci.h> > > > >>>>>>> #include "fuse_i.h" > > > >>>>>>> > > > >>>>>>> -enum { > > > >>>>>>> - /* PCI BAR number of the virtio-fs DAX window */ > > > >>>>>>> - VIRTIO_FS_WINDOW_BAR = 2, > > > >>>>>>> -}; > > > >>>>>>> - > > > >>>>>>> /* List of virtio-fs device instances and a lock for the list */ > > > >>>>>>> static DEFINE_MUTEX(virtio_fs_mutex); > > > >>>>>>> static LIST_HEAD(virtio_fs_instances); > > > >>>>>>> @@ -518,7 +513,7 @@ static int virtio_fs_setup_dax(struct > > > >>>>>>> virtio_device *vdev, struct virtio_fs *fs) > > > >>>>>>> struct dev_pagemap *pgmap; > > > >>>>>>> struct pci_dev *pci_dev; > > > >>>>>>> phys_addr_t phys_addr; > > > >>>>>>> - size_t len; > > > >>>>>>> + size_t bar_len; > > > >>>>>>> int ret; > > > >>>>>>> u8 have_cache, cache_bar; > > > >>>>>>> u64 cache_offset, cache_len; > > > >>>>>>> @@ -551,17 +546,13 @@ static int virtio_fs_setup_dax(struct > > > >>>>>>> virtio_device *vdev, struct virtio_fs *fs) > > > >>>>>>> } > > > >>>>>>> > > > >>>>>>> /* TODO handle case where device doesn't expose BAR? */ > > > >>>>>> > > > >>>>>> For virtio-pmem we decided to not go via BARs as this would > > > >>>>>> effectively > > > >>>>>> make it only usable for virtio-pci implementers. Instead, we are > > > >>>>>> going > > > >>>>>> to export the applicable physical device region directly (e.g. > > > >>>>>> phys_start, phys_size in virtio config), so it is decoupled from > > > >>>>>> PCI > > > >>>>>> details. Doing the same for virtio-fs would allow e.g. also > > > >>>>>> virtio-ccw > > > >>>>>> to make eventually use of this. > > > >>>>> > > > >>>>> That makes it a very odd looking PCI device; I can see that with > > > >>>>> virtio-pmem it makes some sense, given that it's job is to expose > > > >>>>> arbitrary chunks of memory. > > > >>>>> > > > >>>>> Dave > > > >>>> > > > >>>> Well, the fact that your are > > > >>>> > > > >>>> - including <uapi/linux/virtio_pci.h> > > > >>>> - adding pci related code > > > >>>> > > > >>>> in/to fs/fuse/virtio_fs.c > > > >>>> > > > >>>> tells me that these properties might be better communicated on the > > > >>>> virtio layer, not on the PCI layer. > > > >>>> > > > >>>> Or do you really want to glue virtio-fs to virtio-pci for all > > > >>>> eternity? > > > >>> > > > >>> No, these need cleaning up; and the split within the bar > > > >>> is probably going to change to be communicated via virtio layer > > > >>> rather than pci capabilities. However, I don't want to make our PCI > > > >>> device look odd, just to make portability to non-PCI devices - so it's > > > >>> right to make the split appropriately, but still to use PCI bars > > > >>> for what they were designed for. > > > >>> > > > >>> Dave > > > >> > > > >> Let's discuss after the cleanup. In general I am not convinced this is > > > >> the right thing to do. Using virtio-pci for anything else than pure > > > >> transport smells like bad design to me (well, I am no virtio expert > > > >> after all ;) ). No matter what PCI bars were designed for. If we can't > > > >> get the same running with e.g. virtio-ccw or virtio-whatever, it is > > > >> broken by design (or an addon that is tightly glued to virtio-pci, if > > > >> that is the general idea). > > > > > > > > I'm sure we can find alternatives for virtio-*, so I wouldn't expect > > > > it to be glued to virtio-pci. > > > > > > > > Dave > > > > > > As s390x does not have the concept of memory mapped io (RAM is RAM, > > > nothing else), this is not architectured. vitio-ccw can therefore not > > > define anything similar like that. However, in virtual environments we > > > can do whatever we want on top of the pure transport (e.g. on the virtio > > > layer). > > > > > > Conny can correct me if I am wrong. > > > > I don't think you're wrong, but I haven't read the code yet and I'm > > therefore not aware of the purpose of this BAR. > > > > Generally, if there is a memory location shared between host and guest, > > we need a way to communicate its location, which will likely differ > > between transports. For ccw, I could imagine a new channel command > > dedicated to exchanging configuration information (similar to what > > exists today to communicate the locations of virtqueues), but I'd > > rather not go down this path. > > > > Without reading the code/design further, can we use one of the > > following instead of a BAR: > > - a virtqueue; > > - something in config space? > > That would be implementable by any virtio transport. > > The way I think about this is that we wish to extend the VIRTIO device > model with the concept of shared memory. virtio-fs, virtio-gpu, and > virtio-vhost-user all have requirements for shared memory. > > This seems like a transport-level issue to me. PCI supports > memory-mapped I/O and that's the right place to do it. If you try to > put it into config space or the virtqueue, you'll end up with something > that cannot be realized as a PCI device because it bypasses PCI bus > address translation. > > If CCW needs a side-channel, that's fine. But that side-channel is a > CCW-specific mechanism and probably doesn't apply to all other > transports. But virtio-gpu works with ccw right now (I haven't checked what it uses); can virtio-fs use an equivalent method? If there's a more generic case to be made for extending virtio devices with a way to handle shared memory, a ccw for that would be fine. I just want to avoid adding new ccws for everything as the namespace is not infinite.
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