Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram

[Marcel Apfelbaum]

On 01/02/2018 19:36, Eduardo Habkost wrote:
> On Thu, Feb 01, 2018 at 07:12:45PM +0200, Michael S. Tsirkin wrote:
>> On Thu, Feb 01, 2018 at 03:01:36PM -0200, Eduardo Habkost wrote:
>>> On Thu, Feb 01, 2018 at 06:59:07PM +0200, Michael S. Tsirkin wrote:
>>>> On Thu, Feb 01, 2018 at 02:57:39PM -0200, Eduardo Habkost wrote:
>>>>> On Thu, Feb 01, 2018 at 06:48:54PM +0200, Michael S. Tsirkin wrote:
>>>>>> On Thu, Feb 01, 2018 at 02:31:32PM -0200, Eduardo Habkost wrote:
>>>>>>> On Thu, Feb 01, 2018 at 04:24:30PM +0200, Michael S. Tsirkin wrote:
>>>>>>> [...]
>>>>>>>> The full fix would be to allow QEMU to map a list of
>>>>>>>> pages to a guest supplied IOVA.
>>>>>>>
>>>>>>> Thanks, that's what I expected.
>>>>>>>
>>>>>>> While this is not possible, the only requests I have for this
>>>>>>> patch is that we clearly document:
>>>>>>> * What's the only purpose of share=on on a host-memory-backend
>>>>>>>   object (due to pvrdma limitations).
>>>>>>> * The potential undesirable side-effects of setting share=on.
>>>>>>> * On the commit message and other comments, clearly distinguish
>>>>>>>   HVAs in the QEMU address-space from IOVAs, to avoid confusion.
>>>>>>
>>>>>> Looking forward, when we do support it, how will management find out
>>>>>> it no longer needs to pass the share parameter?
>>>>>>
>>>>>> Further, if the side effects of the share parameter go away,
>>>>>> how will it know these no longer hold?
>>>>>
>>>>> A query-host-capabilities or similar QMP command seems necessary
>>>>> for that.
>>>>
>>>> Is anyone working on that?
>>>
>>> Not yet.
>>>
>>> -- 
>>> Eduardo
>>
>> Do these patches need to wait until we do have that command?
> 
> I don't think so.  The command will be needed only when
> support for pvrdma without share=on gets implemented.
> 
> Right now, all we need is clear documentation.
> 
>>
>> I'm thinking it's better to have "share=on required with rdma"
>> and "hugetlbfs not supported with rdma"
>> than the reverse, this way new hosts do not need to carry
>> thus stuff around forever.
> 
> What do you mean by "the reverse"?
> 
> IIUC, the requirements/limitations are:
> 
> * share=on required for pvrdma.  Already documented and enforced
>   by pvrdma code in this series.

Right.

> * hugetlbfs not supported with rdma. Is this detected/reported by
>   QEMU?  Is it documented?

Yes, enforced by the pvrdma device initialization and documented in the
corresponding pvrdma doc.

> * side-effects of share=on.  This is not detected nor documented,
>   and probably already applies to other memory backends.
>   * Nice to have: document when share=on is useful (answer:
>     because of pvrdma), when adding share=on support to
>     host-memory-backend.
> 

The documentation is part of the pvrdma doc.
What are the side-effects of share=on? I missed that.
(share=on is new for the memory backed RAM, the file
backed RAM already had the share parameter)

One can just grep for "share=on" in the docs directory
and can easily see the only current usage. But maybe will
be more, maybe we don't want to limit it for now.

I am planning to re-spin today/tomorrow before sending
a pull-request, can you please point me on what documentation
to add and what side-effects I should document?

Thanks,
Marcel

>>
>> Also, how does management know which devices are affected?
> 
> Right now?  By reading documentation.
>