From f9311031713ab8f18e9c872a42a8f6a9935954ec Mon Sep 17 00:00:00 2001
From: David Gibson <david@gibson.dropbear.id.au>
Date: Wed, 27 Nov 2024 14:54:04 +1100
Subject: [PATCH] iov: iov tail helpers
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.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
iov.c | 93 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
iov.h | 76 ++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 169 insertions(+)
diff --git a/iov.c b/iov.c
index 3741db2..4c6416c 100644
--- a/iov.c
+++ b/iov.c
@@ -155,3 +155,96 @@ size_t iov_size(const struct iovec *iov, size_t iov_cnt)
return len;
}
+
+/**
+ * iov_tail_prune() - Remove any unneeded buffers from an IOV tail
+ * @tail: IO vector tail (modified)
+ *
+ * If an IOV tail's offset is large enough, it may not include any bytes from
+ * the first (or first several) buffers in the underlying IO vector. Modify the
+ * tail's representation so it contains the same logical bytes, but only
+ * includes buffers that are actually needed. This will avoid stepping through
+ * unnecessary elements of the underlying IO vector on future operations.
+ *
+ * Return: true if the tail still contains any bytes, otherwise false
+ */
+bool iov_tail_prune(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_prune(tail);
+ return iov_size(tail->iov, tail->cnt) - tail->off;
+}
+
+/**
+ * iov_peek_header_() - Get pointer to a header from an IOV tail
+ * @tail: IOV tail to get header from
+ * @len: Length of header to get, in bytes
+ * @align: Required alignment of header, in bytes
+ *
+ * @tail may be pruned, but will represent the same bytes as before.
+ *
+ * Returns: Pointer to the first @len logical bytes of the tail, NULL if that
+ * overruns the IO vector, is not contiguous or doesn't have the
+ * requested alignment.
+ */
+void *iov_peek_header_(struct iov_tail *tail, size_t len, size_t align)
+{
+ char *p;
+
+ if (!iov_tail_prune(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 */
+
+ p = (char *)tail->iov[0].iov_base + tail->off;
+ if ((uintptr_t)p % align)
+ return NULL; /* not aligned */
+
+ return p;
+}
+
+/**
+ * iov_remove_header_() - Remove a header from an IOV tail
+ * @tail: IOV tail to remove header from (modified)
+ * @len: Length of header to remove, in bytes
+ * @align: Required alignment of header, in bytes
+ *
+ * On success, @tail is updated so that it longer includes the bytes of the
+ * returned header.
+ *
+ * Returns: Pointer to the first @len logical bytes of the tail, NULL if that
+ * overruns the IO vector, is not contiguous or doesn't have the
+ * requested alignment.
+ */
+/* cppcheck-suppress unusedFunction */
+void *iov_remove_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;
+ return p;
+}
diff --git a/iov.h b/iov.h
index a9e1722..9855bf0 100644
--- a/iov.h
+++ b/iov.h
@@ -28,4 +28,80 @@ 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);
+
+/*
+ * DOC: Theory of Operation, struct iov_tail
+ *
+ * Sometimes a single logical network frame is split across multiple buffers,
+ * represented by an IO vector (struct iovec[]). We often want to process this
+ * one header / network layer at a time. So, it's useful to maintain a "tail"
+ * of the vector representing the parts we haven't yet extracted.
+ *
+ * The headers we extract need not line up with buffer boundaries (though we do
+ * assume they're contiguous within a single buffer for now). So, we could
+ * represent that tail as another struct iovec[], but that would mean copying
+ * the whole array of struct iovecs, just so we can adjust the offset and length
+ * on the first one.
+ *
+ * So, instead represent the tail as pointer into an existing struct iovec[],
+ * with an explicit offset for where the "tail" starts within it. If we extract
+ * enough headers that some buffers of the original vector no longer contain
+ * part of the tail, we (lazily) advance our struct iovec * to the first buffer
+ * we still need, and adjust the vector length and offset to match.
+ */
+
+/**
+ * struct iov_tail - An IO vector which may have some headers logically removed
+ * @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;
+};
+
+/**
+ * IOV_TAIL() - Create a new IOV tail
+ * @iov_: IO vector to create tail from
+ * @cnt_: Length of the IO vector at @iov_
+ * @off_: Byte offset in the IO vector where the tail begins
+ */
+#define IOV_TAIL(iov_, cnt_, off_) \
+ (struct iov_tail){ .iov = (iov_), .cnt = (cnt_), .off = (off_) }
+
+bool iov_tail_prune(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);
+void *iov_remove_header_(struct iov_tail *tail, size_t len, size_t align);
+
+/**
+ * IOV_PEEK_HEADER() - Get typed pointer to a header from an IOV tail
+ * @tail_: IOV tail to get header from
+ * @type_: Data type of the header
+ *
+ * @tail_ may be pruned, but will represent the same bytes as before.
+ *
+ * Returns: Pointer of type (@type_ *) located at the start of @tail_, NULL if
+ * we can't get a contiguous and aligned pointer.
+ */
+#define IOV_PEEK_HEADER(tail_, type_) \
+ ((type_ *)(iov_peek_header_((tail_), \
+ sizeof(type_), __alignof__(type_))))
+
+/**
+ * IOV_REMOVE_HEADER() - Remove and return typed header from an IOV tail
+ * @tail_: IOV tail to remove header from (modified)
+ * @type_: Data type of the header to remove
+ *
+ * On success, @tail_ is updated so that it longer includes the bytes of the
+ * returned header.
+ *
+ * Returns: Pointer of type (@type_ *) located at the old start of @tail_, NULL
+ * if we can't get a contiguous and aligned pointer.
+ */
+#define IOV_REMOVE_HEADER(tail_, type_) \
+ ((type_ *)(iov_remove_header_((tail_), \
+ sizeof(type_), __alignof__(type_))))
+
#endif /* IOVEC_H */