On Tue, Jul 28, 2026 at 11:52:46AM -0400, Peter Xu wrote:
> In an unlikely case, when a migration stream is attached to the destination
> QEMU and only send <4 bytes to the channel as magic, it's possible that
> migration_channel_read_peek() may spin forever without yielding in the main
> thread causing two unwanted consequences:
>
> - CPU will spin 100% waiting for the rest bytes until it reaches 4
> - (more importantly..) Main thread is stuck during this process as the qio
> operation won't really yield the coroutine
>
> Fix it by consuming the bytes that arrived.
>
> Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3889
> Cc: Daniel P. Berrangé <[email protected]>
> Reported-by: Feifan Qian <[email protected]>
> Signed-off-by: Peter Xu <[email protected]>
> ---
> migration/channel.c | 26 +++++++++++++++++++++++---
> 1 file changed, 23 insertions(+), 3 deletions(-)
>
> diff --git a/migration/channel.c b/migration/channel.c
> index 1e2935f926..28fe1d2906 100644
> --- a/migration/channel.c
> +++ b/migration/channel.c
> @@ -294,11 +294,31 @@ int migration_channel_read_peek(QIOChannel *ioc,
> return -1;
> }
>
> - if (len == buflen) {
> + if (len == iov.iov_len) {
> break;
> - }
> + } else if (len == 0) {
> + qio_channel_wait_cond(ioc, G_IO_IN);
> + } else {
> + ssize_t received = len;
>
> - qio_channel_wait_cond(ioc, G_IO_IN);
> + /*
> + * Partially arrived, read out to make qio_channel_wait_cond()
> + * won't return immediately, causing an unwanted spin on this
> + * CPU.
> + */
> + iov.iov_len = len;
> + len = qio_channel_readv_full(ioc, &iov, 1, NULL, NULL, 0, errp);
Sure this breaks the API behaviour that the caller is expecting to
see.
migration_channel_identify will call migration_channel_read_peek
to match the magic bytes.
But something later in the flow will actually try to read the magic
bytes. By consuming them in this migration_channel_read_peek
method, surely we're breaking the code that wants to read the bytes
later.
> + /*
> + * QIO_CHANNEL_ERR_BLOCK also shouldn't happen, due to the
> + * prior peek just happened. We should be pretty sure we will
> + * read what we peeked, or the channel was broken.
> + */
> + if (len != received) {
> + return -1;
> + }
> + iov.iov_base += received;
> + iov.iov_len = buflen - received;
> + }
> }
>
> return 0;
> --
> 2.54.0
>
With regards,
Daniel
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