From: Pekka Enberg <penberg@kernel.org>
Date: Fri, 8 Apr 2011 11:53:26 +0000 (+0300)
Subject: kvm tools: Use pread() and pwrite() in disk-image.c
X-Git-Tag: next-20110824~3^2~480
X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=df3663f30d9b85f6a7692783c569b9b8415cdf56;p=karo-tx-linux.git

kvm tools: Use pread() and pwrite() in disk-image.c

This patch convert lseek() + read()/write() pairs with pread()/pwrite(). The
performance of running the following command

  dd if=/dev/vda of=/dev/null

in the guest does not show significant performance differences but should be a
net win because of the reduction in system calls.

Signed-off-by: Pekka Enberg <penberg@kernel.org>
---

diff --git a/tools/kvm/disk-image.c b/tools/kvm/disk-image.c
index 9a830270c683..80999c13d505 100644
--- a/tools/kvm/disk-image.c
+++ b/tools/kvm/disk-image.c
@@ -35,10 +35,7 @@ static int raw_image__read_sector(struct disk_image *self, uint64_t sector, void
 	if (offset + dst_len > self->size)
 		return -1;
 
-	if (lseek(self->fd, offset, SEEK_SET) < 0)
-		return -1;
-
-	if (read_in_full(self->fd, dst, dst_len) < 0)
+	if (pread_in_full(self->fd, dst, dst_len, offset) < 0)
 		return -1;
 
 	return 0;
@@ -51,10 +48,7 @@ static int raw_image__write_sector(struct disk_image *self, uint64_t sector, voi
 	if (offset + src_len > self->size)
 		return -1;
 
-	if (lseek(self->fd, offset, SEEK_SET) < 0)
-		return -1;
-
-	if (write_in_full(self->fd, src, src_len) < 0)
+	if (pwrite_in_full(self->fd, src, src_len, offset) < 0)
 		return -1;
 
 	return 0;
diff --git a/tools/kvm/include/kvm/read-write.h b/tools/kvm/include/kvm/read-write.h
index a49394b021ff..069326b2a40d 100644
--- a/tools/kvm/include/kvm/read-write.h
+++ b/tools/kvm/include/kvm/read-write.h
@@ -10,4 +10,10 @@ ssize_t xwrite(int fd, const void *buf, size_t count);
 ssize_t read_in_full(int fd, void *buf, size_t count);
 ssize_t write_in_full(int fd, const void *buf, size_t count);
 
+ssize_t xpread(int fd, void *buf, size_t count, off_t offset);
+ssize_t xpwrite(int fd, const void *buf, size_t count, off_t offset);
+
+ssize_t pread_in_full(int fd, void *buf, size_t count, off_t offset);
+ssize_t pwrite_in_full(int fd, const void *buf, size_t count, off_t offset);
+
 #endif /* KVM_READ_WRITE_H */
diff --git a/tools/kvm/read-write.c b/tools/kvm/read-write.c
index 46b147d3b8b6..398918b5fe99 100644
--- a/tools/kvm/read-write.c
+++ b/tools/kvm/read-write.c
@@ -77,3 +77,78 @@ ssize_t write_in_full(int fd, const void *buf, size_t count)
 
 	return total;
 }
+
+/* Same as pread(2) except that this function never returns EAGAIN or EINTR. */
+ssize_t xpread(int fd, void *buf, size_t count, off_t offset)
+{
+	ssize_t nr;
+
+restart:
+	nr = pread(fd, buf, count, offset);
+	if ((nr < 0) && ((errno == EAGAIN) || (errno == EINTR)))
+		goto restart;
+
+	return nr;
+}
+
+/* Same as pwrite(2) except that this function never returns EAGAIN or EINTR. */
+ssize_t xpwrite(int fd, const void *buf, size_t count, off_t offset)
+{
+	ssize_t nr;
+
+restart:
+	nr = pwrite(fd, buf, count, offset);
+	if ((nr < 0) && ((errno == EAGAIN) || (errno == EINTR)))
+		goto restart;
+
+	return nr;
+}
+
+ssize_t pread_in_full(int fd, void *buf, size_t count, off_t offset)
+{
+	ssize_t total = 0;
+	char *p = buf;
+
+	while (count > 0) {
+		ssize_t nr;
+
+		nr = xpread(fd, p, count, offset);
+		if (nr <= 0) {
+			if (total > 0)
+				return total;
+
+			return -1;
+		}
+
+		count -= nr;
+		total += nr;
+		p += nr;
+		offset += nr;
+	}
+
+	return total;
+}
+
+ssize_t pwrite_in_full(int fd, const void *buf, size_t count, off_t offset)
+{
+	const char *p = buf;
+	ssize_t total = 0;
+
+	while (count > 0) {
+		ssize_t nr;
+
+		nr = xpwrite(fd, p, count, offset);
+		if (nr < 0)
+			return -1;
+		if (nr == 0) {
+			errno = ENOSPC;
+			return -1;
+		}
+		count -= nr;
+		total += nr;
+		p += nr;
+		offset += nr;
+	}
+
+	return total;
+}