On Wed, Nov 06, 2024 at 11:06:18AM +0100, Stefano Brivio wrote:On Wed, 6 Nov 2024 13:38:38 +1100 David Gibson <david(a)gibson.dropbear.id.au> wrote:On Wed, Nov 06, 2024 at 01:56:31AM +0100, Stefano Brivio wrote:Now that I read your reply, the "tail" name becomes clearer as it's clear that we just use it for left-overs. I don't think "remainder" makes it much clearer, so I would rather stick to "tail" and try to explain the purpose of struct iov_tail a bit more specifically (see below).On Tue, 5 Nov 2024 13:32:22 +1100 David Gibson <david(a)gibson.dropbear.id.au> wrote:Yeah, I don't love the names either.In the vhost-user code we have a number of places where we need to locate a particular header within the guest-supplied IO vector. We need to work out which buffer the header is in, and verify that it's contiguous and aligned as we need. At the moment this is open-coded, but introduce a helper to make this more straightforward. We add a new datatype 'struct iov_tail' representing an IO vector from which we've logically consumed some number of headers. The IOV_PULL_HEADER macro consumes a new header from the vector, returning a pointer and updating the iov_tail.The interfaces look usable and straightforward to me. I find some names and comments a bit obscure, though.First off, I would intuitively say that the "tail" is always at the end, and if we already consumed something, that's always at the "head".Right.. which is true in a sense. The idea is you'd set one of these up, to cover a whole (say) frame, then pull bits off the front as you need it. So, the iov_tail does represent the "tail", as in the unprocessed bit of the frame at each point...If we call the whole abstraction "tail", we risk ending up talking about the tail of the tail, and the head of the tail. Consider this part from the cover letter:.. but, yeah, heads of tails and tails of tails gets confusing. Unless we rewrite in LISP, I guess.I don't think "batch" is really any better, it's just unclear on a different set of axes. Would "iov remainder" be any better?"iov tail", that is an iov from which you've already consumed (in some sense) some data from the beginning....in other words, that's an IO vector called tail, and we already consumed some data from its head. What about (iov-based) "batch"?Good idea. I've gone with "prune".Oh, this wasn't clear to me. I guess that "reduce", "minimise", or "prune" would convey this better.Yeah, I'm not sure how to express this either. In a sense this operation is a logical no-op: it shouldn't change the results of any future operation, but it might make them slightly faster.Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au> --- iov.c | 83 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ iov.h | 24 +++++++++++++++++ 2 files changed, 107 insertions(+) diff --git a/iov.c b/iov.c index 3f9e229..3d384ae 100644 --- a/iov.c +++ b/iov.c @@ -156,3 +156,86 @@ size_t iov_size(const struct iovec *iov, size_t iov_cnt) return len; } + +/** + * iov_tail_shorten() - Remove any buffers from an IOV tail that are wholly consumed"Remove" is a bit difficult to interpret (does it deallocate? Throw data away^), I would rather say that we... detach (?) those buffers from the batch/tail.Eh, barely, even then. We'd still need to have a fall back path for when the header is *really* not contiguous. So it's a tiny optimization for a case that will only happen if we're improbably lucky about where the buffers end up. Incidentally it would be quite easy to extend these helpers to handle the split header case. Add a parameter with a "spare" buffer of the necessary size/type. If the header is contiguous we just return it as now, otherwise we linearize it into the spare buffer and return a pointer to that. Not optimal, but might be worth it for the simplicity."Head block"? Or forget about it, "header" is actually fine.This operation itself raises a question though: if the batch already carries the information that some buffers were completely consumed, should it ever be in a state where we want to drop these buffers from it? That is, it sounds like we have some other operation that allows it to be in an inconsistent state.Sort of, yes, but I think this is the right design choice. It means we can trivially construct one of these things with an arbitrary offset, and we only do the work of stepping through the buffers when we actually have to. We can discard bytes simply by adding to the offset, without having to look at the actual buffers until later. Basically if we want to ensure that the representation is always "minimal", in the sense that the offset lies within the first buffer, then a bunch more operations need to do work to maintain that. Plus, this approach is naturally robust: if we somehow get an iov_tail in non-minimal form, peek/pull will just handle it with no extra logic.Hm. Well, we are using it for headers, and we're also always pulling from the "head" of whatever we have left.+ * @tail: IO vector tail (modified) + * + * Return: true if the tail still contains any bytes, otherwise false + */ +bool iov_tail_shorten(struct iov_tail *tail) +{ + size_t i; + + i = iov_skip_bytes(tail->iov, tail->cnt, tail->off, &tail->off); + tail->iov += i; + tail->cnt -= i; + + return !!tail->cnt; +} + +/** + * iov_tail_size - Calculate the total size of an IO vector tail + * @tail: IO vector tail + * + * Returns: The total size in bytes. + */ +/* cppcheck-suppress unusedFunction */ +size_t iov_tail_size(struct iov_tail *tail) +{ + iov_tail_shorten(tail); + return iov_size(tail->iov, tail->cnt) - tail->off; +} + +/** + * iov_peek_header_() - Get pointer to header from an IOV tailI think that this needs to be more generic than "header", because yes, we're using it for headers, but that word doesn't really help in this context.What about "aligned block", or just "block"?Maybe... but it really does have to be from the start of the current tail.I guess it's worth it if there's any chance we'll ever want to support split headers.Hm, true. Not sure if it's worth the tests handle that case though.+ * @tail: IO vector tail to get header from + * @len: Length of header to remove in bytesto remove, in bytes+ * @align: Required alignment of header in bytesJudging from this comment alone, it's not clear if 0 or 1 should be used to get freely aligned blocks.+ * + * @tail may be modified, but will be semantically equivalent. + * + * Returns: Pointer to the removed header, NULL if it overruns the IO + * vector, is not contiguous or is misaligned. + */ +void *iov_peek_header_(struct iov_tail *tail, size_t len, size_t align) +{ + char *p; + + if (!iov_tail_shorten(tail)) + return NULL; /* Nothing left */ + + if (tail->off + len < tail->off) + return NULL; /* Overflow */ + + if (tail->off + len > tail->iov[0].iov_len) + return NULL; /* Not contiguous */I'm not sure if this observation is useful in some cases, but this doesn't necessarily mean that the header/block is not contiguous: if tail->iov[0].iov_base + tail->iov[0].iov_len == tail->iov[1].iov_base, it actually is.> > > + p = (char *)tail->iov[0].iov_base + tail->off; > > > + if ((uintptr_t)p % align) > > > + return NULL; /* not aligned */ > > > + > > > + return p; > > > +} > > > +/** > > > + * iov_pull_header_() - Remove a header from an IOV tail > > > > I know that "pulling" is widely used, but it's sometimes ambiguous (I > > guess we already had a discussion about that in the past). What about > > "remove", or "drop"? > > "remove" might work. I don't like "drop" because that implies to me > it's just gone, rather than returned.I'm going with "remove" for now.Ok. I've added a "theory of operation" comment, as well as revising a bunch of other comments, which I hope will help....so, it's not just a tail portion, it's a tail portion in a rather specific and constrained context, and once that's revealed these helpers are easier (for me) to understand. What about: * struct iov_tail - Remaining, not consumed portion (tail) of IO vector ?True. > > + return p; > > +} > > diff --git a/iov.h b/iov.h > > index a9e1722..a2f449c 100644 > > --- a/iov.h > > +++ b/iov.h > > @@ -28,4 +28,28 @@ size_t iov_from_buf(const struct iovec *iov, size_t iov_cnt, > > size_t iov_to_buf(const struct iovec *iov, size_t iov_cnt, > > size_t offset, void *buf, size_t bytes); > > size_t iov_size(const struct iovec *iov, size_t iov_cnt); > > + > > +/** > > + * struct iov_tail - Represents the fail portion of an IO vector+ * @tail: IO vector tail to remove header from (modified) + * @len: Length of header to remove in bytes + * @align: Required alignment of header in bytes + * + * @tail is updated so that it no longer includes the extracted header + * + * Returns: Pointer to the removed header, NULL if it overruns the IO + * vector, is not contiguous or is misaligned. + */ +/* cppcheck-suppress unusedFunction */ +void *iov_pull_header_(struct iov_tail *tail, size_t len, size_t align) +{ + char *p = iov_peek_header_(tail, len, align); + + if (!p) + return NULL; + + tail->off = tail->off + len;This could just be += I guess.I've actually given them full function-style comments. -- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibsonOh. That's kind of obscure. I guess it would be better to actually spell that out and define this on multiple lines.s/fail/tail/"type"+ * @iov: IO vector + * @cnt: Number of entries in @iov + * @off: Current offset in @iov + */ +struct iov_tail { + const struct iovec *iov; + size_t cnt, off; +}; + +#define IOV_TAIL(iov_, cnt_, off_) \ + (struct iov_tail){ .iov = (iov_), .cnt = (cnt_), .off = (off_) } + +bool iov_tail_shorten(struct iov_tail *tail); +size_t iov_tail_size(struct iov_tail *tail); +void *iov_peek_header_(struct iov_tail *tail, size_t len, size_t align); +#define IOV_PEEK_HEADER(tail_, ty_) \ + ((ty_ *)(iov_peek_header_((tail_), sizeof(ty_), __alignof__(ty_))))I guess 'x' would be as clear as 'ty_' (actually, I'm failing to guess what it stands for).I see now, I didn't pay much attention to that. Perhaps some one-liner function-like comments around IOV_TAIL(), IOV_PEEK_HEADER() and IOV_PULL_HEADER() would help.The IOV_TAIL() macro builds one. Adding data doesn't make sense; the idea is this is a view into part of an existing IO vector, which doesn't require copying the struct iovecs of that original vector.+void *iov_pull_header_(struct iov_tail *tail, size_t len, size_t align); +#define IOV_PULL_HEADER(tail_, ty_) \ + ((ty_ *)(iov_pull_header_((tail_), sizeof(ty_), __alignof__(ty_)))) + #endif /* IOVEC_H */I would have expected some functions to add data or build those tails... or we don't need them for some reason?