* David Hildenbrand (david@xxxxxxxxxx) wrote: > On 15.05.20 19:51, Dr. David Alan Gilbert wrote: > > * David Hildenbrand (david@xxxxxxxxxx) wrote: > >> On 15.05.20 14:45, Dr. David Alan Gilbert wrote: > >>> * David Hildenbrand (david@xxxxxxxxxx) wrote: > >>>> RDMA will pin all guest memory (as documented in docs/rdma.txt). We want > >>>> to mark RAM block discards to be broken - however, to keep it simple > >>>> use ram_block_discard_is_required() instead of inhibiting. > >>> > >>> Should this be dependent on whether rdma->pin_all is set? > >>> Even with !pin_all some will be pinned at any given time > >>> (when it's registered with the rdma stack). > >> > >> Do you know how much memory this is? Is such memory only temporarily pinned? > > > > With pin_all not set, only a subset of memory, I think multiple 1MB > > chunks, are pinned at any one time. > > > >> At least with special-cases of vfio, it's acceptable if some memory is > >> temporarily pinned - we assume it's only the working set of the driver, > >> which guests will not inflate as long as they don't want to shoot > >> themselves in the foot. > >> > >> This here sounds like the guest does not know the pinned memory is > >> special, right? > > > > Right - for RDMA it's all of memory that's being transferred, and the > > guest doesn't see when each part is transferred. > > > Okay, so all memory will eventually be pinned, just not at the same > time, correct? > > I think this implies that any memory that was previously discarded will > be backed my new pages, meaning we will consume more memory than intended. > > If so, always disabling discarding of RAM seems to be the right thing to do. Yeh that's probably true, although there's a check for 'buffer_is_zero' in the !rdma->pin_all case, if the entire area is zero (or probably if unmapped) then it sends a notification rather than registering; see qemu_rdma_write_one and search for 'This chunk has not yet been registered, so first check to see' Dave > > -- > Thanks, > > David / dhildenb -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
