On Wed, Sep 10, 2025 at 05:31:56PM +0300, Vladimir Sementsov-Ogievskiy wrote:
In migration we want to pass fd "as is", not changing its
blocking status.
The only current user of these fds is CPR state (through VMSTATE_FD),
which of-course doesn't want to modify fds on target when source is
still running and use these fds.
Suggested-by: Daniel P. Berrangé <[email protected]>
Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
---
v3: RESEND, add qemu-devel to CC, sorry for the noise
v2: rework, following Daniel's suggestion to use flag.
include/io/channel.h | 1 +
io/channel-socket.c | 12 ++++++++----
io/channel.c | 2 +-
migration/qemu-file.c | 3 ++-
4 files changed, 12 insertions(+), 6 deletions(-)
diff --git a/include/io/channel.h b/include/io/channel.h
index 234e5db70d..5394f50768 100644
--- a/include/io/channel.h
+++ b/include/io/channel.h
@@ -36,6 +36,7 @@ OBJECT_DECLARE_TYPE(QIOChannel, QIOChannelClass,
#define QIO_CHANNEL_READ_FLAG_MSG_PEEK 0x1
#define QIO_CHANNEL_READ_FLAG_RELAXED_EOF 0x2
+#define QIO_CHANNEL_READ_FLAG_PRESERVE_BLOCKING 0x4
Shouldn't the name reflect FD somehow? Or it can imply preservation
blocking for the channel itself.
QIO_CHANNEL_READ_FLAG_FD_PRESERVE_BLOCKING
QIO_CHANNEL_READ_FLAG_PRESERVE_FD_BLOCKING
...
typedef enum QIOChannelFeature QIOChannelFeature;
diff --git a/io/channel-socket.c b/io/channel-socket.c
index 3b7ca924ff..2f6e2d84a3 100644
--- a/io/channel-socket.c
+++ b/io/channel-socket.c
@@ -464,7 +464,8 @@ static void qio_channel_socket_finalize(Object *obj)
#ifndef WIN32
static void qio_channel_socket_copy_fds(struct msghdr *msg,
- int **fds, size_t *nfds)
+ int **fds, size_t *nfds,
+ bool preserve_blocking)
{
struct cmsghdr *cmsg;
@@ -497,8 +498,10 @@ static void qio_channel_socket_copy_fds(struct msghdr *msg,
continue;
}
- /* O_NONBLOCK is preserved across SCM_RIGHTS so reset it */
- qemu_socket_set_block(fd);
+ if (!preserve_blocking) {
+ /* O_NONBLOCK is preserved across SCM_RIGHTS so reset it */
+ qemu_socket_set_block(fd);
+ }
#ifndef MSG_CMSG_CLOEXEC
qemu_set_cloexec(fd);
@@ -556,7 +559,8 @@ static ssize_t qio_channel_socket_readv(QIOChannel *ioc,
}
if (fds && nfds) {
- qio_channel_socket_copy_fds(&msg, fds, nfds);
+ qio_channel_socket_copy_fds(
+ &msg, fds, nfds, flags & QIO_CHANNEL_READ_FLAG_PRESERVE_BLOCKING);
}
return ret;
diff --git a/io/channel.c b/io/channel.c
index ebd9322765..50d5f7b10b 100644
--- a/io/channel.c
+++ b/io/channel.c
@@ -58,7 +58,7 @@ ssize_t qio_channel_readv_full(QIOChannel *ioc,
{
QIOChannelClass *klass = QIO_CHANNEL_GET_CLASS(ioc);
- if ((fds || nfds) &&
+ if ((fds || nfds || (flags & QIO_CHANNEL_READ_FLAG_PRESERVE_BLOCKING)) &&
Not a huge deal, but.. IMHO we can simply ignore this flag when fds==NULL.
It can also make callers' lives slightly easier too by always passing in
this flag when necessary, like in below.
!qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_FD_PASS)) {
error_setg_errno(errp, EINVAL,
"Channel does not support file descriptor passing");
diff --git a/migration/qemu-file.c b/migration/qemu-file.c
index b6ac190034..92c7b5678b 100644
--- a/migration/qemu-file.c
+++ b/migration/qemu-file.c
@@ -324,6 +324,7 @@ static ssize_t coroutine_mixed_fn qemu_fill_buffer(QEMUFile
*f)
size_t nfd = 0;
int **pfds = f->can_pass_fd ? &fds : NULL;
size_t *pnfd = f->can_pass_fd ? &nfd : NULL;
+ int flags = f->can_pass_fd ? QIO_CHANNEL_READ_FLAG_PRESERVE_BLOCKING : 0;
assert(!qemu_file_is_writable(f));
@@ -340,7 +341,7 @@ static ssize_t coroutine_mixed_fn qemu_fill_buffer(QEMUFile *f)
do {
struct iovec iov = { f->buf + pending, IO_BUF_SIZE - pending };
- len = qio_channel_readv_full(f->ioc, &iov, 1, pfds, pnfd, 0,
+ len = qio_channel_readv_full(f->ioc, &iov, 1, pfds, pnfd, flags,
&local_error);
if (len == QIO_CHANNEL_ERR_BLOCK) {
if (qemu_in_coroutine()) {
--
2.48.1
