From: Olaf Hering <olaf@aepfle.de>
Date: Wed, 28 Oct 2020 12:07:36 +0100
Subject: libxc sr readv_exact

tools: add readv_exact to libxenctrl

Read a batch of iovec's.

Short reads are the common case, finish the trailing iov with read_exact.

Signed-off-by: Olaf Hering <olaf@aepfle.de>

v2:
- add comment to short-read handling
---
 tools/libs/ctrl/xc_private.c | 57 +++++++++++++++++++++++++++++++++++-
 tools/libs/ctrl/xc_private.h |  1 +
 2 files changed, 57 insertions(+), 1 deletion(-)

--- a/tools/libs/ctrl/xc_private.c
+++ b/tools/libs/ctrl/xc_private.c
@@ -699,8 +699,23 @@ int write_exact(int fd, const void *data
 
 #if defined(__MINIOS__)
 /*
- * MiniOS's libc doesn't know about writev(). Implement it as multiple write()s.
+ * MiniOS's libc doesn't know about readv/writev().
+ * Implement it as multiple read/write()s.
  */
+int readv_exact(int fd, const struct iovec *iov, int iovcnt)
+{
+    int rc, i;
+
+    for ( i = 0; i < iovcnt; ++i )
+    {
+        rc = read_exact(fd, iov[i].iov_base, iov[i].iov_len);
+        if ( rc )
+            return rc;
+    }
+
+    return 0;
+}
+
 int writev_exact(int fd, const struct iovec *iov, int iovcnt)
 {
     int rc, i;
@@ -715,6 +730,46 @@ int writev_exact(int fd, const struct io
     return 0;
 }
 #else
+int readv_exact(int fd, const struct iovec *iov, int iovcnt)
+{
+    int rc = 0, idx = 0;
+    ssize_t len;
+
+    while ( idx < iovcnt )
+    {
+        len = readv(fd, &iov[idx], min(iovcnt - idx, IOV_MAX));
+        if ( len == -1 && errno == EINTR )
+            continue;
+        if ( len <= 0 )
+        {
+            rc = -1;
+            goto out;
+        }
+
+        /* Finish a potential short read in the last iov */
+        while ( len > 0 && idx < iovcnt )
+        {
+            if ( len >= iov[idx].iov_len )
+            {
+                len -= iov[idx].iov_len;
+            }
+            else
+            {
+                void *p = iov[idx].iov_base + len;
+                size_t l = iov[idx].iov_len - len;
+
+                rc = read_exact(fd, p, l);
+                if ( rc )
+                    goto out;
+                len = 0;
+            }
+            idx++;
+        }
+    }
+out:
+    return rc;
+}
+
 int writev_exact(int fd, const struct iovec *iov, int iovcnt)
 {
     struct iovec *local_iov = NULL;
--- a/tools/libs/ctrl/xc_private.h
+++ b/tools/libs/ctrl/xc_private.h
@@ -389,6 +389,7 @@ int xc_flush_mmu_updates(xc_interface *x
 
 /* Return 0 on success; -1 on error setting errno. */
 int read_exact(int fd, void *data, size_t size); /* EOF => -1, errno=0 */
+int readv_exact(int fd, const struct iovec *iov, int iovcnt);
 int write_exact(int fd, const void *data, size_t size);
 int writev_exact(int fd, const struct iovec *iov, int iovcnt);