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. > 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. This will fix rdma with hugetlbfs as well which is currently broken. > -- > Eduardo