On Tue, Jul 28, 2026 at 05:24:36PM +0100, Daniel P. Berrangé wrote:
> 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.
Ah right, stupid me. The best then is for partial read we apply a manual
wait.
I can also revert 604bb1badc ("migration: Properly wait on G_IO_IN when
peeking messages"), looping with 1ms interval for data isn't too bad. But
the best is we only do that for partial, so:
diff --git a/migration/channel.c b/migration/channel.c
index 1e2935f926..88f89521f8 100644
--- a/migration/channel.c
+++ b/migration/channel.c
@@ -296,9 +296,19 @@ int migration_channel_read_peek(QIOChannel *ioc,
if (len == buflen) {
break;
+ } else if (len == 0) {
+ qio_channel_wait_cond(ioc, G_IO_IN);
+ } else {
+ /*
+ * When partially ready, we can't use qio_channel_wait_cond()
+ * because it will return immediately. Apply a manual wait.
+ */
+ if (qemu_in_coroutine()) {
+ qemu_co_sleep_ns(QEMU_CLOCK_REALTIME, 1000000);
+ } else {
+ g_usleep(1000);
+ }
}
-
- qio_channel_wait_cond(ioc, G_IO_IN);
}
return 0;
Any preference?
Thanks,
>
> > + /*
> > + * 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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>
--
Peter Xu
