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 12:26:29 +0100
From:   David Hildenbrand <david@...hat.com>
To:     "Dr. David Alan Gilbert" <dgilbert@...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

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).

-- 

Thanks,

David / dhildenb