On Thu, Feb 01, 2018 at 07:36:50AM +0200, Marcel Apfelbaum wrote: > On 01/02/2018 4:22, Michael S. Tsirkin wrote: > > On Wed, Jan 31, 2018 at 09:34:22PM -0200, Eduardo Habkost wrote: > >> On Wed, Jan 31, 2018 at 11:10:07PM +0200, Michael S. Tsirkin wrote: > >>> On Wed, Jan 31, 2018 at 06:40:59PM -0200, Eduardo Habkost wrote: > >>>> On Wed, Jan 17, 2018 at 11:54:18AM +0200, Marcel Apfelbaum wrote: > >>>>> Currently only file backed memory backend can > >>>>> be created with a "share" flag in order to allow > >>>>> sharing guest RAM with other processes in the host. > >>>>> > >>>>> Add the "share" flag also to RAM Memory Backend > >>>>> in order to allow remapping parts of the guest RAM > >>>>> to different host virtual addresses. This is needed > >>>>> by the RDMA devices in order to remap non-contiguous > >>>>> QEMU virtual addresses to a contiguous virtual address range. > >>>>> > >>>> > >>>> Why do we need to make this configurable? Would anything break > >>>> if MAP_SHARED was always used if possible? > >>> > >>> See Documentation/vm/numa_memory_policy.txt for a list > >>> of complications. > >> > >> Ew. > >> > >>> > >>> Maybe we should more of an effort to detect and report these > >>> issues. > >> > >> Probably. Having other features breaking silently when using > >> pvrdma doesn't sound good. We must at least document those > >> problems in the documentation for memory-backend-ram. > >> > >> BTW, what's the root cause for requiring HVAs in the buffer? > > > > It's a side effect of the kernel/userspace API which always wants > > a single HVA/len pair to map memory for the application. > > > > > > Hi Eduardo and Michael, > > >> Can > >> this be fixed? > > > > I think yes. It'd need to be a kernel patch for the RDMA subsystem > > mapping an s/g list with actual memory. The HVA/len pair would then just > > be used to refer to the region, without creating the two mappings. > > > > Something like splitting the register mr into > > > > mr = create mr (va/len) - allocate a handle and record the va/len > > > > addmemory(mr, offset, hva, len) - pin memory > > > > register mr - pass it to HW > > > > As a nice side effect we won't burn so much virtual address space. > > > > We would still need a contiguous virtual address space range (for post-send) > which we don't have since guest contiguous virtual address space > will always end up as non-contiguous host virtual address space.
It just needs to be contiguous in the HCA virtual address space. Software never accesses through this pointer. In other words - basically expose register physical mr to userspace. > > I am not sure the RDMA HW can handle a large VA with holes. > > An alternative would be 0-based MR, QEMU intercepts the post-send > operations and can substract the guest VA base address. > However I didn't see the implementation in kernel for 0 based MRs > and also the RDMA maintainer said it would work for local keys > and not for remote keys. > > > This will fix rdma with hugetlbfs as well which is currently broken. > > > > > > There is already a discussion on the linux-rdma list: > https://www.spinics.net/lists/linux-rdma/msg60079.html > But it will take some (actually a lot of) time, we are currently talking about > a possible API. You probably need to pass the s/g piece by piece since it might exceed any reasonable array size. > And it does not solve the re-mapping... > > Thanks, > Marcel Haven't read through that discussion. But at least what I posted solves it since you do not need it contiguous in HVA any longer. > >> -- > >> Eduardo