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

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Date:   Thu, 13 Dec 2018 10:00:59 +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 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

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