We observed slow responses on a host forwarded port on Windows. Investigation revealed that qemu_fd_register was being called with fd=-1 and this caused g_poll in qemu_poll_ns timing out. I tracked this behavior to following commit:
748f8f4 slirp: replace qemu_set_nonblock()

@@ -482,7 +483,8 @@ void tcp_connect(struct socket *inso)
         tcp_close(sototcpcb(so)); /* This will sofree() as well */
         return;
     }
-    qemu_set_nonblock(s);
+    slirp_set_nonblock(s);
+    so->slirp->cb->register_poll_fd(so->s);

It seems that calling register_poll_fd with so->s instead of s may be a typo. Changing it back to s solves this issue. The commit 748f8f4 made similar change in tcp_fconnect but I have not touched it.

Signed-off-by: Hafiz Abid Qadeer <abidh@codesourcery.com>
---
 src/tcp_subr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/tcp_subr.c b/src/tcp_subr.c
index a1016d9..1b6d602 100644
--- a/src/tcp_subr.c
+++ b/src/tcp_subr.c
@@ -493,7 +493,7 @@ void tcp_connect(struct socket *inso)
         return;
     }
     slirp_set_nonblock(s);
-    so->slirp->cb->register_poll_fd(so->s, so->slirp->opaque);
+    so->slirp->cb->register_poll_fd(s, so->slirp->opaque);
     slirp_socket_set_fast_reuse(s);
     opt = 1;
     setsockopt(s, SOL_SOCKET, SO_OOBINLINE, &opt, sizeof(int));
-- 
2.25.1

On Fri, Jan 29, 2021 at 02:31:49PM +0000, Hafiz Abid Qadeer wrote:
> We observed slow responses on a host forwarded port on Windows. Investigation revealed that qemu_fd_register was being called with fd=-1 and this caused g_poll in qemu_poll_ns timing out. I tracked this behavior to following commit:
> 748f8f4 slirp: replace qemu_set_nonblock()
> 
> @@ -482,7 +483,8 @@ void tcp_connect(struct socket *inso)
>          tcp_close(sototcpcb(so)); /* This will sofree() as well */
>          return;
>      }
> -    qemu_set_nonblock(s);
> +    slirp_set_nonblock(s);
> +    so->slirp->cb->register_poll_fd(so->s);
> 
> It seems that calling register_poll_fd with so->s instead of s may be a typo. Changing it back to s solves this issue. The commit 748f8f4 made similar change in tcp_fconnect but I have not touched it.
> 
> Signed-off-by: Hafiz Abid Qadeer <abidh@codesourcery.com>
> ---
>  src/tcp_subr.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Thanks for your suggested patch, however, slirp is now spun off into its
own standalone project. You can submit patches using merge requests at

  https://gitlab.freedesktop.org/slirp/libslirp

QEMU will periodically pull in updates from slirp.

Regards,
Daniel
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