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From: David Hildenbrand <david@redhat.com>
To: Cornelia Huck <cohuck@redhat.com>, Stefan Hajnoczi <stefanha@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: Tue, 18 Dec 2018 18:25:27 +0100
Message-ID: <4a870d9c-8c46-7f2e-4947-a3913a824d14@redhat.com> (raw)
In-Reply-To: <20181218181358.3bc2615a.cohuck@redhat.com>

On 18.12.18 18:13, Cornelia Huck wrote:
> On Mon, 17 Dec 2018 14:56:38 +0000
> Stefan Hajnoczi <stefanha@redhat.com> wrote:
> 
>> On Mon, Dec 17, 2018 at 11:53:46AM +0100, David Hildenbrand wrote:
>>> 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:
> 
>>>>>> 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?  
>>
>> If you implement a physical virtio PCI adapter then there is bus
>> addressing and an IOMMU and VIRTIO has support for that.  I'm not sure I
>> understand what you mean by "there is no iommu"?
> 
> For ccw, there is no iommu; channel-program translation is doing
> similar things. (I hope that is what David meant :)
> 
>>
>>> 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.  
>>
>> virtio-gpu currently does not use shared memory, it needs it for future
>> features.
> 
> OK, that all sounds like we need to define a generic, per transport,
> device agnostic way to specify shared memory.
> 
> Where is that memory situated? Is it something in guest memory (like
> virtqueues)? If it is something provided by the device, things will get
> tricky for ccw (remember that there's no mmio on s390; pci on s390 uses
> special instructions for that.)
> 

I am just very very confused right now. What I am struggling with right
now (Stefan, hope you can clarify it for me):

We need some place where this shared memory is located in the guest
physical memory. On x86 - if I am not wrong - this BAR is placed into
the reserved memory area between 3 and 4 GB. There is no such thing on
s390x. Because we don't have IO via memory (yet). All we have is one or
two KVM memory slots filled with all memory.

So what we will need on s390x is on the QEMU side such a reserved memory
region where devices like virtio-fs can reserve a region for shared memory.

So it is something like a dimm/nvdimm except that it is smaller and not
visible to the user directly (via memory backends).

-- 

Thanks,

David / dhildenb

  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
2018-12-17 14:56                       ` Stefan Hajnoczi
2018-12-18 17:13                         ` Cornelia Huck
2018-12-18 17:25                           ` David Hildenbrand [this message]
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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