Date:   Tue, 18 Dec 2018 18:25:27 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Cornelia Huck <cohuck@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>
Cc:     "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        miklos@...redi.hu, sweil@...hat.com, swhiteho@...hat.com
Subject: Re: [PATCH 18/52] virtio-fs: Map cache using the values from the
 capabilities

On 18.12.18 18:13, Cornelia Huck wrote:
> On Mon, 17 Dec 2018 14:56:38 +0000
> Stefan Hajnoczi <stefanha@...hat.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@...hat.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

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