On 20.05.2022 14:37, Demi Marie Obenour wrote: > On Fri, May 20, 2022 at 08:24:48AM +0200, Jan Beulich wrote: >> On 20.05.2022 01:22, Demi Marie Obenour wrote: >>> It is well known that mapping and unmapping grants is expensive, which >>> is why blkback has persistent grants. Could this cost be mitigated by >>> batching, and if it was, would it affect the tradeoff of memcpy() vs >>> grant table operations? >> >> Which backend driver are you thinking about? The in-kernel Linux >> xen-blkback already batches grant operations, afaics. Such >> batching is helpful, but the main cost is assumed (known?) to be >> with the (installing and) tearing down of the actual mappings of >> the guest pages (into/)from backend address space. > > My thought was that the expensive part of this is TLB flushes, which are > only needed once per batch.
Correct, but as said - such batching is already being made use of by the in-kernel Linux backend. Of course "once per batch" is not entirely precise - very large batches would be split internally into smaller chunks, each of which would be followed by a TLB flush. > Also, what do you think about the “unsafe” > mode? It would only be unsafe if the backend is untrusted, but it is > quite common for the backend to be trusted. Well, I didn't put much thought into that (and hence intentionally didn't comment on it), so for now it's only "Why not?" Jan
