On Tue, Feb 12, 2013 at 09:08:27PM +0100, Paolo Bonzini wrote: > Il 12/02/2013 19:23, Michael S. Tsirkin ha scritto: > > On Tue, Feb 12, 2013 at 07:04:27PM +0100, Paolo Bonzini wrote: > >>>> Perhaps, but 3 or 4 arguments (in/out/nsg or in/out/nsg_in/nsg_out) just > >>>> for this are definitely too many and make the API harder to use. > >>>> > >>>> You have to find a balance. Having actually used the API, the > >>>> possibility of mixing in/out buffers by mistake never even occurred to > >>>> me, much less happened in practice, so I didn't consider it a problem. > >>>> Mixing in/out buffers in a single call wasn't a necessity, either. > >>> > >>> It is useful for virtqueue_add_buf implementation. > >> > >> ret = virtqueue_start_buf(vq, data, out + in, !!out + !!in, > >> gfp); > >> if (ret < 0) > >> return ret; > >> > >> if (out) > >> virtqueue_add_sg(vq, sg, out, DMA_TO_DEVICE); > >> if (in) > >> virtqueue_add_sg(vq, sg + out, in, DMA_FROM_DEVICE); > >> > >> virtqueue_end_buf(vq); > >> return 0; > >> > >> How can it be simpler and easier to understand than that? > > > > Like this: > > > > ret = virtqueue_start_buf(vq, data, in, out, gfp); > > if (ret < 0) > > return ret; > > > > virtqueue_add_sg(vq, sg, in, out); > > > > virtqueue_end_buf(vq); > > It's out/in, not in/out... I know you wrote it in a hurry, but it kind > of shows that the new API is easier to use. Check out patch 8, it's a > real improvement in readability. That's virtqueue_add_buf_single, that's a separate matter. Another option for _single is just two wrappers: virtqueue_add_buf_in virtqueue_add_buf_out > Plus you haven't solved the problem of alternating to/from-device > elements (which is also harder to spot with in/out than with the enum). Yes it does, if add_sg does not have in/out at all there's no way to request the impossible to/from mix. > And no one else would use add_sg with in != 0 && out != 0, so you'd be > favoring one caller over all the others. Yes but it's an important caller as all drivers besides storage use this path. > If you did this instead: > > virtqueue_add_sg(vq, sg, in + out); > > it would really look like a hack IMHO. > > >>> Basically the more consistent the interface is with virtqueue_add_buf, > >>> the better. > >> > >> The interface is consistent with virtqueue_add_buf_single, where out/in > >> clearly doesn't make sense. > > > > Hmm, we could make virtqueue_add_buf_single consistent by giving it 'bool in'. > > But is it "bool in" or "bool out"? Agree, bool is a bit ugly anyway. > >> virtqueue_add_buf and virtqueue_add_sg are very different, despite the > >> similar name. > > > > True. The similarity is between _start and _add_buf. > > And this is confusing too. Maybe this means > > _start and _add_sg should be renamed. > > Maybe. If you have any suggestions it's fine. > > BTW I tried using out/in for start_buf, and the code in virtio-blk gets > messier, it has to do all the math twice. I'm pretty sure we can do this without duplication, if we want to. > Perhaps we just need to > acknowledge that the API is different and thus the optimal choice of > arguments is different. C doesn't have keyword arguments, there not > much that we can do. > > Paolo Yea, maybe. I'm not the API guru here anyway, it's Rusty's street. -- MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
