linux-kernel - Re: [PATCH 18/52] virtio-fs: Map cache using the values from the capabilities

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Message-ID: <20181213121548.GN2313@work-vm>
Date:   Thu, 13 Dec 2018 12:15:49 +0000
From:   "Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Vivek Goyal <vgoyal@...hat.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        miklos@...redi.hu, stefanha@...hat.com, sweil@...hat.com,
        swhiteho@...hat.com
Subject: Re: [PATCH 18/52] virtio-fs: Map cache using the values from the
 capabilities

* David Hildenbrand (david@...hat.com) wrote:
> On 13.12.18 11:00, Dr. David Alan Gilbert wrote:
> > * David Hildenbrand (david@...hat.com) wrote:
> >> On 13.12.18 10:13, Dr. David Alan Gilbert wrote:
> >>> * David Hildenbrand (david@...hat.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 <dgilbert@...hat.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

> -- 
> 
> Thanks,
> 
> David / dhildenb
--
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK