On 07/25/2012 03:49 PM, Paolo Bonzini wrote: > > Except here the destination array has to be given to virtio, which > doesn't (yet) understand chaining. I'm using for_each_sg rather than a > simple memcpy exactly because I want to flatten the input scatterlist > onto consecutive scatterlist entries, which is what virtio expects (and > what I'll change when I get to it). > > for_each_sg guarantees that I get non-chain scatterlists only, so it is > okay to value-assign them to sg[]. > So if the virtio does not understand chaining at all then surly it will not understand the 2-bit end marker and will get a wrong page pointer with the 1st bit set. As I said!! Since the last code did sg_set_buff() and worked then you must change it with sg_set_page(). If there are *any* chaining-or-no-chaining markers errors these should be fixed as a separate patch! Please lets concentrate at the problem at hand. <snip> > > It was _not_ properly terminated, and didn't matter because virtio > doesn't care about termination. Changing all the virtio devices to > properly terminate chains (and to use for_each_sg) is a prerequisite for > properly supporting long sglists). > Fine then your code is now a crash because the terminating bit was just copied over, which it was not before. ------------ Now Back to the how to support chaining: Lets separate the two topics from now on. Send me one mail concerning the proper above patch, And a different mail for how to support chaining. >> In SCSI land most LLDs should support chaining just by virtu of using the >> for_each_sg macro. That all it takes. Your code above does support it. > > Yes, it supports it but still has to undo them before passing to virtio. > > What my LLD does is add a request descriptor in front of the scatterlist > that the LLD receives. I would like to do this with a 2-item > scatterlist: one for the request descriptor, and one which is a chain to > the original scatterlist. I hate that plan. Why yet override the scatter element yet again with a third union of a "request descriptor" The reason it was overloaded as a link-pointer in the first place was because of historical compatibility reasons and not because of a good design. You should have a proper "request descriptor" structure defined, pointing to (or followed by), an sglist-chain. And all of the above is mute. > Except that if I call sg_chain and my > architecture does not define ARCH_HAS_SG_CHAIN, it will BUG out. So I > need to keep all the scatterlist allocation and copying crap that I have > now within #ifdef, and it will bitrot. > except that with the correct design you don't call sg_chain you just do: request_descriptor->sg_list = sg; >>> I would need to add support for the long sglists to virtio; this is not >>> a problem, but in the past Rusty complained that long sg-lists are not >>> well suited to virtio (which would like to add elements not just at the >>> beginning of a given sglist, but also at the end). >> >> Well that can be done as well, (If done carefully) It should be easy to add >> chained fragments to both the end of a given chain just as at beginning. >> It only means that the last element of the appended-to chain moves to >> the next fragment and it's place is replaced by a link. > > But you cannot do that in constant time, can you? And that means you do > not enjoy any benefit in terms of cache misses etc. > I did not understand "constant time" it is O(0) if that what you meant. (And surly today's code of copy the full list "cache misses") > Also, this assumes that I can modify the appended-to chain. I'm not > sure I can do this? > Each case it's own. If the appended-to chain is const, yes you'll have to reallocate it and copy. Is that your case? Cheers Boaz >>> It seems to me that virtio would prefer to work with a struct >>> scatterlist ** rather than a long sglist. >> >> That's just going backwards, and lazy. As you said if you want to enjoy >> the better performance cake you better break some eggs ;-) > > :) > > Paolo -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
