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From: David Hildenbrand <david@redhat.com>
To: Stefan Hajnoczi <stefanha@redhat.com>, Cornelia Huck <cohuck@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Vivek Goyal <vgoyal@redhat.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, miklos@szeredi.hu, sweil@redhat.com,
	swhiteho@redhat.com
Subject: Re: [PATCH 18/52] virtio-fs: Map cache using the values from the capabilities
Date: Mon, 17 Dec 2018 11:53:46 +0100
Message-ID: <cf910f77-203c-8efd-fed1-6b041b078d63@redhat.com> (raw)
In-Reply-To: <20181214134434.GA3882@stefanha-x1.localdomain>

On 14.12.18 14:44, Stefan Hajnoczi 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 <david@redhat.com> 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 <dgilbert@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.
> 
> Stefan
> 

I think the problem is more fundamental. There is no iommu. Whatever
shared region you want to indicate, you want it to be assigned a memory
region in guest physical memory. Like a DIMM/NVDIMM. And this should be
different to the concept of a BAR. Or am I missing something?

I am ok with using whatever other channel to transport such information.
But I believe this is different to a typical BAR. (I wish I knew more
about PCI internals ;) ).

I would also like to know how shared memory works as of now for e.g.
virtio-gpu.

-- 

Thanks,

David / dhildenb

  parent reply index

Thread overview: 98+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-12-10 17:12 [PATCH 00/52] [RFC] virtio-fs: shared file system for virtual machines Vivek Goyal
2018-12-10 17:12 ` [PATCH 01/52] fuse: add skeleton virtio_fs.ko module Vivek Goyal
2018-12-10 17:12 ` [PATCH 02/52] fuse: add probe/remove virtio driver Vivek Goyal
2018-12-10 17:12 ` [PATCH 03/52] fuse: rely on mutex_unlock() barrier instead of fput() Vivek Goyal
2018-12-10 17:12 ` [PATCH 04/52] fuse: extract fuse_fill_super_common() Vivek Goyal
2018-12-10 17:12 ` [PATCH 05/52] virtio_fs: get mount working Vivek Goyal
2018-12-10 17:12 ` [PATCH 06/52] fuse: export fuse_end_request() Vivek Goyal
2018-12-10 17:12 ` [PATCH 07/52] fuse: export fuse_len_args() Vivek Goyal
2018-12-10 17:12 ` [PATCH 08/52] fuse: add fuse_iqueue_ops callbacks Vivek Goyal
2018-12-10 17:12 ` [PATCH 09/52] fuse: process requests queues Vivek Goyal
2018-12-10 17:12 ` [PATCH 10/52] fuse: export fuse_get_unique() Vivek Goyal
2018-12-10 17:12 ` [PATCH 11/52] fuse: implement FUSE_FORGET for virtio-fs Vivek Goyal
2018-12-10 17:12 ` [PATCH 12/52] virtio_fs: Set up dax_device Vivek Goyal
2018-12-10 17:12 ` [PATCH 13/52] dax: remove block device dependencies Vivek Goyal
2018-12-10 17:12 ` [PATCH 14/52] fuse: add fuse_conn->dax_dev field Vivek Goyal
2018-12-10 17:12 ` [PATCH 15/52] fuse: map virtio_fs DAX window BAR Vivek Goyal
2018-12-12 16:37   ` Christian Borntraeger
2018-12-13 11:55     ` Stefan Hajnoczi
2018-12-13 16:06   ` kbuild test robot
2018-12-13 19:55   ` Dan Williams
2018-12-13 20:09     ` Dr. David Alan Gilbert
2018-12-13 20:15       ` Dan Williams
2018-12-13 20:40         ` Vivek Goyal
2018-12-13 21:18           ` Vivek Goyal
2018-12-14 10:09             ` Dr. David Alan Gilbert
2018-12-10 17:12 ` [PATCH 16/52] virtio-fs: Add VIRTIO_PCI_CAP_SHARED_MEMORY_CFG and utility to find them Vivek Goyal
2018-12-12 16:36   ` [PATCH] virtio-fs: fix semicolon.cocci warnings kbuild test robot
2018-12-12 16:36   ` [PATCH 16/52] virtio-fs: Add VIRTIO_PCI_CAP_SHARED_MEMORY_CFG and utility to find them kbuild test robot
2018-12-10 17:12 ` [PATCH 17/52] virtio-fs: Retrieve shm capabilities for cache Vivek Goyal
2018-12-10 17:12 ` [PATCH 18/52] virtio-fs: Map cache using the values from the capabilities Vivek Goyal
2018-12-13  9:10   ` David Hildenbrand
2018-12-13  9:13     ` Dr. David Alan Gilbert
2018-12-13  9:34       ` David Hildenbrand
2018-12-13 10:00         ` Dr. David Alan Gilbert
2018-12-13 11:26           ` David Hildenbrand
2018-12-13 12:15             ` Dr. David Alan Gilbert
2018-12-13 12:24               ` David Hildenbrand
2018-12-13 12:38                 ` Cornelia Huck
2018-12-14 13:44                   ` Stefan Hajnoczi
2018-12-14 13:50                     ` Cornelia Huck
2018-12-14 14:06                       ` Dr. David Alan Gilbert
2018-12-17 11:25                       ` Stefan Hajnoczi
2018-12-17 10:53                     ` David Hildenbrand [this message]
2018-12-17 14:56                       ` Stefan Hajnoczi
2018-12-18 17:13                         ` Cornelia Huck
2018-12-18 17:25                           ` David Hildenbrand
2019-01-02 10:24                             ` Stefan Hajnoczi
2019-03-17  0:33   ` Liu Bo
2019-03-20 10:42     ` Dr. David Alan Gilbert
2019-03-17  0:35   ` [PATCH] virtio-fs: fix multiple tag support Liu Bo
2019-03-19 20:26     ` Vivek Goyal
2019-03-20  2:04       ` Liu Bo
2018-12-10 17:12 ` [PATCH 19/52] virito-fs: Make dax optional Vivek Goyal
2018-12-10 17:12 ` [PATCH 20/52] Limit number of pages returned by direct_access() Vivek Goyal
2018-12-10 17:12 ` [PATCH 21/52] fuse: Introduce fuse_dax_mapping Vivek Goyal
2018-12-10 17:12 ` [PATCH 22/52] Create a list of free memory ranges Vivek Goyal
2018-12-11 17:44   ` kbuild test robot
2018-12-15 19:22   ` kbuild test robot
2018-12-10 17:12 ` [PATCH 23/52] fuse: simplify fuse_fill_super_common() calling Vivek Goyal
2018-12-10 17:12 ` [PATCH 24/52] fuse: Introduce setupmapping/removemapping commands Vivek Goyal
2018-12-10 17:12 ` [PATCH 25/52] Introduce interval tree basic data structures Vivek Goyal
2018-12-10 17:12 ` [PATCH 26/52] fuse: Implement basic DAX read/write support commands Vivek Goyal
2018-12-10 17:12 ` [PATCH 27/52] fuse: Maintain a list of busy elements Vivek Goyal
2018-12-10 17:12 ` [PATCH 28/52] Do fallocate() to grow file before mapping for file growing writes Vivek Goyal
2018-12-11  6:13   ` kbuild test robot
2018-12-11  6:20   ` kbuild test robot
2018-12-10 17:12 ` [PATCH 29/52] fuse: add DAX mmap support Vivek Goyal
2018-12-10 17:12 ` [PATCH 30/52] fuse: delete dentry if timeout is zero Vivek Goyal
2018-12-10 17:12 ` [PATCH 31/52] dax: Pass dax_dev to dax_writeback_mapping_range() Vivek Goyal
2018-12-11  6:12   ` kbuild test robot
2018-12-11 17:38   ` kbuild test robot
2018-12-10 17:12 ` [PATCH 32/52] fuse: Define dax address space operations Vivek Goyal
2018-12-10 17:12 ` [PATCH 33/52] fuse, dax: Take ->i_mmap_sem lock during dax page fault Vivek Goyal
2018-12-10 17:13 ` [PATCH 34/52] fuse: Add logic to free up a memory range Vivek Goyal
2018-12-10 17:13 ` [PATCH 35/52] fuse: Add logic to do direct reclaim of memory Vivek Goyal
2018-12-10 17:13 ` [PATCH 36/52] fuse: Kick worker when free memory drops below 20% of total ranges Vivek Goyal
2018-12-10 17:13 ` [PATCH 37/52] fuse: multiplex cached/direct_io/dax file operations Vivek Goyal
2018-12-10 17:13 ` [PATCH 38/52] Dispatch FORGET requests later instead of dropping them Vivek Goyal
2018-12-10 17:13 ` [PATCH 39/52] Release file in process context Vivek Goyal
2018-12-10 17:13 ` [PATCH 40/52] fuse: Do not block on inode lock while freeing memory range Vivek Goyal
2018-12-10 17:13 ` [PATCH 41/52] fuse: Reschedule dax free work if too many EAGAIN attempts Vivek Goyal
2018-12-10 17:13 ` [PATCH 42/52] fuse: Wait for memory ranges to become free Vivek Goyal
2018-12-10 17:13 ` [PATCH 43/52] fuse: Take inode lock for dax inode truncation Vivek Goyal
2018-12-10 17:13 ` [PATCH 44/52] fuse: Clear setuid bit even in direct I/O path Vivek Goyal
2018-12-10 17:13 ` [PATCH 45/52] virtio: Free fuse devices on umount Vivek Goyal
2018-12-10 17:13 ` [PATCH 46/52] virtio-fs: Retrieve shm capabilities for version table Vivek Goyal
2018-12-10 17:13 ` [PATCH 47/52] virtio-fs: Map using the values from the capabilities Vivek Goyal
2018-12-10 17:13 ` [PATCH 48/52] virtio-fs: pass version table pointer to fuse Vivek Goyal
2018-12-10 17:13 ` [PATCH 49/52] fuse: don't crash if version table is NULL Vivek Goyal
2018-12-10 17:13 ` [PATCH 50/52] fuse: add shared version support (virtio-fs only) Vivek Goyal
2018-12-10 17:13 ` [PATCH 51/52] fuse: shared version cleanups Vivek Goyal
2018-12-10 17:13 ` [PATCH 52/52] fuse: fix fuse_permission() for the default_permissions case Vivek Goyal
2018-12-19 21:25   ` kbuild test robot
2018-12-11 12:54 ` [PATCH 00/52] [RFC] virtio-fs: shared file system for virtual machines Stefan Hajnoczi
2018-12-12 20:30 ` Konrad Rzeszutek Wilk
2018-12-12 21:22   ` Vivek Goyal
2019-02-12 15:56 ` Aneesh Kumar K.V
2019-02-12 18:57   ` Vivek Goyal

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