File xen.sr-readv_exact.patch of Package xen

From: Olaf Hering <olaf@aepfle.de>
Date: Wed, 28 Oct 2020 12:07:36 +0100
Subject: 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
@@ -690,40 +690,95 @@ int write_exact(int fd, const void *data, size_t size)
         if ( (len == -1) && (errno == EINTR) )
             continue;
         if ( len <= 0 )
             return -1;
         offset += len;
     }
 
     return 0;
 }
 
 #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;
 
     for ( i = 0; i < iovcnt; ++i )
     {
         rc = write_exact(fd, iov[i].iov_base, iov[i].iov_len);
         if ( rc )
             return rc;
     }
 
     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;
     int rc = 0, iov_idx = 0, saved_errno = 0;
     ssize_t len;
 
     while ( iov_idx < iovcnt )
     {
         /*
          * Skip over iov[] entries with 0 length.
          *
          * This is needed to cover the case where we took a partial write and
--- a/tools/libs/ctrl/xc_private.h
+++ b/tools/libs/ctrl/xc_private.h
@@ -386,24 +386,25 @@ struct xc_mmu {
     mmu_update_t updates[MAX_MMU_UPDATES];
     int          idx;
     unsigned int subject;
 };
 /* Structure returned by xc_alloc_mmu_updates must be free()'ed by caller. */
 struct xc_mmu *xc_alloc_mmu_updates(xc_interface *xch, unsigned int subject);
 int xc_add_mmu_update(xc_interface *xch, struct xc_mmu *mmu,
                    unsigned long long ptr, unsigned long long val);
 int xc_flush_mmu_updates(xc_interface *xch, struct xc_mmu *mmu);
 
 /* 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);
 
 int xc_ffs8(uint8_t x);
 int xc_ffs16(uint16_t x);
 int xc_ffs32(uint32_t x);
 int xc_ffs64(uint64_t x);
 
 #define DOMPRINTF(fmt, args...) xc_dom_printf(dom->xch, fmt, ## args)
 #define DOMPRINTF_CALLED(xch) xc_dom_printf((xch), "%s: called", __FUNCTION__)
 
 /**