On Thu, Dec 13, 2018 at 01:38:23PM +0100, Cornelia Huck wrote: > On Thu, 13 Dec 2018 13:24:31 +0100 > David Hildenbrand wrote: > > > On 13.12.18 13:15, Dr. David Alan Gilbert wrote: > > > * David Hildenbrand (david@redhat.com) wrote: > > >> On 13.12.18 11:00, Dr. David Alan Gilbert wrote: > > >>> * David Hildenbrand (david@redhat.com) wrote: > > >>>> On 13.12.18 10:13, Dr. David Alan Gilbert wrote: > > >>>>> * David Hildenbrand (david@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 > > >>>>>>> --- > > >>>>>>> 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 > > >>>>>>> #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 > > >>>> - 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. Stefan