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. >
