Peter Xu <pet...@redhat.com> writes: > On Mon, Jan 29, 2024 at 09:42:24AM -0300, Fabiano Rosas wrote: >> Peter Xu <pet...@redhat.com> writes: >> >> > On Fri, Jan 26, 2024 at 07:19:39PM -0300, Fabiano Rosas wrote: >> >> +static MultiFDMethods multifd_socket_ops = { >> >> + .send_setup = multifd_socket_send_setup, >> >> + .send_cleanup = multifd_socket_send_cleanup, >> >> + .send_prepare = multifd_socket_send_prepare, >> > >> > Here it's named with "socket", however not all socket-based multifd >> > migrations will go into this route, e.g., when zstd compression enabled it >> > will not go via this route, even if zstd also uses sockets as transport. >> > From that pov, this may be slightly confusing. Maybe it suites more to be >> > called "socket_plain" / "socket_no_comp"? >> > >> > One step back, I had a feeling that the current proposal tried to provide a >> > single ->ops to cover a model where we may need more than one layer of >> > abstraction. >> > >> > Since it might be helpful to allow multifd send arbitrary data (e.g. for >> > VFIO? Avihai might have an answer there..), I'll try to even consider that >> > into the picture. >> > >> > Let's consider the ultimate goal of multifd, where the simplest model could >> > look like this in my mind (I'm only discussing sender side, but it'll be >> > similar on recv side): >> > >> > prepare() send() >> > Input ----------------> IOVs ------------> iochannels >> > >> > [I used prepare/send, but please think them as generic terms, not 100% >> > aligned with what we have with existing multifd_ops, or what you proposed >> > later] >> > >> > Here what are sure, IMHO, is: >> > >> > - We always can have some input data to dump; I didn't use "guest pages" >> > just to say we may allow arbitrary data. For any multifd user that >> > would like to dump arbitrary data, they can already provide IOVs, so >> > here input can be either "MultiFDPages_t" or "IOVs". >> >> Or anything else, since the client code also has control over send(), >> no? So it could give multifd a pointer to some memory and then use >> send() to do whatever it wants with it. Multifd is just providing worker >> threads and "scheduling". > > IOVs contain the case of one single buffer, where n_iovs==1. Here I > mentioned IOVs explicitly because I want to make it part of the protocol so > that the interface might be clearer, on what is not changing, and what can > change for a multifd client.
Got it. I agree. >> >> Also note that multifd clients currently _do not_ provide IOVs. They >> merely provide data to multifd (p->pages) and then convert that data >> into IOVs at prepare(). This is different, because multifd currently >> holds that p->pages (and turns that into p->normal), which means the >> client code does not need to store the data across iterations (in the >> case of RAM which is iterative). > > They provide? AFAIU that's exactly MultiFDSendParams.iov as of now, while > iov_nums is the length. Before that, the ram code needs to pass in the p->pages->offset array first. Then, that gets put into p->normal. Then, that gets put into p->iov at prepare(). So it's not a simple "fill p->iov and pass it to multifd". Hmm, could we just replace multifd_send_state->pages with a multifd_send_state->iov? I don't really understand why do we need to carry that pages->offset around. >> >> > >> > - We may always want to have IOVs to represent the buffers at some point, >> > no matter what the input it >> > >> > - We always flush the IOVs to iochannels; basically I want to say we can >> > always assume the last layer is connecting to QIOChannel APIs, while I >> > don't think there's outliers here so far, even if the send() may >> > differ. >> > >> > Then _maybe_ it's clearer that we can have two layers of OPs? >> > >> > - prepare(): it tells how the "input" will be converted into a scatter >> > gatter list of buffers. All compression methods fall into this afaiu. >> > This has _nothing_ to do on how the buffers will be sent. For >> > arbitrary-typed input, this can already be a no-op since the IOVs >> > provided can already be passed over to send(). >> > >> > - send(): how to dump the IOVs to the iochannels. AFAIU this is motly >> > only useful for fixed-ram migrations. >> > >> > Would this be clearer, rather than keep using a single multifd_ops? >> >> Sorry, I don't see how what you describe is any different than what we >> have. And I don't see how any of this would mean more than one >> multifd_ops. We already have multifd_ops->prepare() and >> multifd_ops->send(). What am I missing? > > I meant instead of having a single MultiFDMethods, we can have > MultifdPrepareOps and MultifdSendOps separately. > > Now with single MultiFDMethods, it must always provide e.g. both prepare() > and send() in one set of OPs for one use case. What I wanted to say is > maybe it is cleaner we split it into two OPs, then all the socket-based > scenarios can already stick with the same send() method, even though they > can prepare() differently. Hmm, so zlib/zstd implement all ops except for the send one. And socket_plain and file implement all prepare hooks plus the send. So we'd have sort of a data handling layer and a transport layer. I'll see how it looks. > > IOW, for this base patchset to pave way for compression accelerators, IIUC > we don't need a send() yet so far? Should they still work pretty well with > qio_channel_writev_full_all() with proper touchups on p->write_flags just > for zero copy purposes? Yes. The point here is to just give everyone a heads-up so we avoid changing the code in incompatible ways. > > I'll have a read again to your previous multifd-packet-cleanups branch I > guess. but this series definitely doesn't apply there already. multifd-packet-cleanups attempts to replace MultiFDPages_t with a generic data structure. That's a separate issue.