Most chardev backend handle write() as discarded data if underlying system is disconnected. For unknown historical reasons, the Spice backend has "reliable" write: it will wait until the client end is reconnected to do further successful write(). To decide whether it make sense to wait until the client is reconnected (or queue the writes), let's review Spice chardev usage and handling of a disconnected client: * spice vdagent The agents reopen the virtio port on disconnect. In qemu side, virtio_serial_close() will also discard pending data. * usb redirection A disconnect creates a device disconnection. * smartcard emulation Data is discarded in passthru_apdu_from_guest(). (Spice doesn't explicitly open the smartcard char device until upcoming 0.14.2, commit 69a5cfc74131ec0459f2eb5a231139f5a69a8037) * spice webdavd The daemon will restart the service, and reopen the virtio port. * spice ports (serial console, qemu monitor..) Depends on the associated device or usage. - serial, may be throttled or discarded on write, depending on device - QMP/HMP monitor have some CLOSED event handling, but want to flush the write, which will finish when a new client connects. On disconnect/reconnect, the client starts with fresh sessions. If it is a seamless migration, the client disconnects after the source migrated. The handling of source disconnect in qemu is thus irrelevant for the Spice session migration. For all these use cases, it is better to discard writes when the client is disconnected, and require the vm-side device/agent to behave correctly on CHR_EVENT_CLOSED, to stop reading and writing from the spice chardev. Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com> Tested-by: Victor Toso <victortoso@redhat.com> Message-id: 20190221110703.5775-3-marcandre.lureau@redhat.com Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
20 lines
675 B
Plaintext
20 lines
675 B
Plaintext
# See docs/devel/tracing.txt for syntax documentation.
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# chardev/wctablet.c
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wct_init(void) ""
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wct_cmd_re(void) ""
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wct_cmd_st(void) ""
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wct_cmd_sp(void) ""
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wct_cmd_ts(int input) "0x%02x"
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wct_cmd_other(const char *cmd) "%s"
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wct_speed(int speed) "%d"
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# chardev/spice.c
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spice_chr_discard_write(int len) "spice chr write discarded %d"
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spice_vmc_write(ssize_t out, int len) "spice wrote %zd of requested %d"
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spice_vmc_read(int bytes, int len) "spice read %d of requested %d"
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spice_vmc_register_interface(void *scd) "spice vmc registered interface %p"
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spice_vmc_unregister_interface(void *scd) "spice vmc unregistered interface %p"
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spice_vmc_event(int event) "spice vmc event %d"
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