lib/ukfile: Add utility inlines for iovec I/O

This change adds a utility header providing convenience inlines for
doing I/O on buffers described by struct iovec, namely:
- zero out
- scatter data from buffer to iov
- gather data from iov into buffer

Signed-off-by: Andrei Tatar <andrei@unikraft.io>

lib/ukfile/include/uk/file/iovutil.h

@@ -0,0 +1,171 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
+/* Copyright (c) 2024, Unikraft GmbH and The Unikraft Authors.
+ * Licensed under the BSD-3-Clause License (the "License").
+ * You may not use this file except in compliance with the License.
+ */
+
+/* Convenience operations for dealing with iovec */
+
+#ifndef __UKFILE_IOVUTIL_H__
+#define __UKFILE_IOVUTIL_H__
+
+#include <string.h>
+#include <sys/uio.h>
+
+#include <uk/essentials.h>
+
+/**
+ * Zero out at most `len` bytes in memory regions described by `iov[iovcnt]`,
+ * starting at `*curp` offset from the buffer at `iov[*iovip]`.
+ *
+ * If the total remaining space in `iov` is less than `len`, exit early.
+ * After the call, `*iovip` and `*curp` are updated with the new positions.
+ *
+ * @return Number of bytes zeroed
+ */
+static inline
+size_t uk_iov_zero(const struct iovec *iov, int iovcnt, size_t len,
+		   int *iovip, size_t *curp)
+{
+	size_t ret = 0;
+	int i = *iovip;
+	size_t cur = *curp;
+
+	UK_ASSERT(i < iovcnt);
+	UK_ASSERT(cur < iov[i].iov_len);
+	if (cur) {
+		const size_t l = MIN(len, iov[i].iov_len - cur);
+
+		memset((char *)iov[i].iov_base + cur, 0, l);
+		ret += l;
+		len -= l;
+		cur += l;
+		if (cur == iov[i].iov_len) {
+			i++;
+			cur = 0;
+		}
+	}
+	if (len) {
+		UK_ASSERT(!cur);
+		while (i < iovcnt && len) {
+			const size_t l = MIN(iov[i].iov_len, len);
+
+			memset(iov[i].iov_base, 0, l);
+			ret += l;
+			len -= l;
+			if (len)
+				i++;
+			else
+				cur = l;
+		}
+	}
+	*iovip = i;
+	*curp = cur;
+	return ret;
+}
+
+/**
+ * Copy at most `len` bytes from `buf` into memory regions described by
+ * `iov[iovcnt]`, starting at `*curp` offset from the buffer at `iov[*iovip]`.
+ *
+ * If the total remaining space in `iov` is less than `len`, exit early.
+ * After the call, `*iovip` and `*curp` are updated with the new positions.
+ *
+ * @return Number of bytes copied
+ */
+static inline
+size_t uk_iov_scatter(const struct iovec *iov, int iovcnt, const char *buf,
+		      size_t len, int *iovip, size_t *curp)
+{
+	size_t ret = 0;
+	int i = *iovip;
+	size_t cur = *curp;
+
+	UK_ASSERT(i < iovcnt);
+	UK_ASSERT(cur < iov[i].iov_len);
+	if (cur) {
+		const size_t l = MIN(len, iov[i].iov_len - cur);
+
+		memcpy((char *)iov[i].iov_base + cur, buf, l);
+		ret += l;
+		buf += l;
+		len -= l;
+		cur += l;
+		if (cur == iov[i].iov_len) {
+			i++;
+			cur = 0;
+		}
+	}
+	if (len) {
+		UK_ASSERT(!cur);
+		while (i < iovcnt && len) {
+			const size_t l = MIN(iov[i].iov_len, len);
+
+			memcpy(iov[i].iov_base, buf, l);
+			ret += l;
+			buf += l;
+			len -= l;
+			if (len)
+				i++;
+			else
+				cur = l;
+		}
+	}
+	*iovip = i;
+	*curp = cur;
+	return ret;
+}
+
+/**
+ * Copy at most `len` bytes from the memory regions described by `iov[iovcnt]`,
+ * starting at `*curp` offset from the buffer at `iov[*iovip]`, into `buf`.
+ *
+ * If the total remaining bytes in `iov` are less than `len`, exit early.
+ * After the call, `*iovip` and `*curp` are updated with the new positions.
+ *
+ * @return Number of bytes copied
+ */
+static inline
+size_t uk_iov_gather(char *buf, const struct iovec *iov, int iovcnt,
+		     size_t len, int *iovip, size_t *curp)
+{
+	size_t ret = 0;
+	int i = *iovip;
+	size_t cur = *curp;
+
+	UK_ASSERT(i < iovcnt);
+	UK_ASSERT(cur < iov[i].iov_len);
+	if (cur) {
+		const size_t l = MIN(len, iov[i].iov_len - cur);
+
+		memcpy(buf, (char *)iov[i].iov_base + cur, l);
+		ret += l;
+		buf += l;
+		len -= l;
+		cur += l;
+		if (cur == iov[i].iov_len) {
+			i++;
+			cur = 0;
+		}
+	}
+	if (len) {
+		UK_ASSERT(!cur);
+		while (i < iovcnt && len) {
+			const size_t l = MIN(iov[i].iov_len, len);
+
+			memcpy(buf, iov[i].iov_base, l);
+			ret += l;
+			buf += l;
+			len -= l;
+			if (len)
+				i++;
+			else
+				cur = l;
+		}
+	}
+	*iovip = i;
+	*curp = cur;
+	return ret;
+}
+
+#endif /* __UKFILE_IOVUTIL_H__ */