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

[Eduardo Habkost <ehabkost@redhat.com>]

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

> 
> Also, how does management know which devices are affected?

Right now?  By reading documentation.

-- 
Eduardo