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Merged
baptleduc merged 1 commit into from
Jul 3, 2026
Merged

doc/riscv: detail PV console input path #37

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35 changes: 31 additions & 4 deletions doc/riscv/xen-console.puml
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,8 @@ skinparam participantPadding 20
skinparam boxPadding 10

actor "User" as user
participant "dom0\nhvc0 tty\n(drivers/tty/hvc/hvc_xen.c)" as hvc0
participant "dom0\nxl console\n(tools/xl)" as xlc
participant "dom0\nxenconsoled\n(tools/console/daemon/io.c)" as xcd
participant "Xen\nhypervisor\n(xen/common/)" as xen
participant "domU\nLinux hvc0\n(drivers/tty/hvc/hvc_xen.c)" as domu
Expand Down Expand Up @@ -51,14 +53,39 @@ end note

== Console Input (user types a character) ==

user -> xcd : keystroke on pty master
note over user, xen
INPUT path uses handle_tty_read(), NOT handle_tty_write().
(handle_tty_write is the reverse: guest output → pty master.)
end note

user -> xen : keystroke in QEMU serial\n(Xen owns the physical UART)
xen -> xen : serial_rx() → __serial_rx()\nserial_rx_ring[prod++] = c\nsend_global_virq(VIRQ_CONSOLE)\n(xen/drivers/char/console.c)
xen -> hvc0 : VIRQ_CONSOLE event channel\n(notify hardware domain)
hvc0 -> xen : HYPERVISOR_console_io(CONSOLEIO_read)
xen --> hvc0 : drain serial_rx_ring → char\n(do_console_io, console.c)

note over hvc0
char surfaces on dom0 hvc0 tty (/dev/hvc0)
end note

hvc0 -> xlc : char readable on /dev/hvc0\n(xl console's stdin)

note over xlc
operator runs `xl console <domU>`; it reads the
char from stdin (hvc0) and writes it to the
pty SLAVE (/dev/pts/N, path from xenstore
/local/domain/<domid>/console/tty).
end note

xlc -> xcd : write char to pty slave (/dev/pts/N)\n→ becomes readable on master_fd

note over xcd
handle_tty_write()
tools/console/daemon/io.c
poll() returns POLLIN on master_fd
→ handle_tty_read()
tools/console/daemon/io.c:1019
end note

xcd -> xcd : write char to in[] ring\nintf->in[in_prod++ & mask] = char\nxen_mb()
xcd -> xcd : read(master_fd, msg)\nwrite char to in[] ring\nintf->in[in_prod++ & mask] = char\nxen_wmb()\nintf->in_prod = prod
xcd -> xen : xenevtchn_notify(local_port)\n[SBI ecall → evtchn_send()]

note over xen
Expand Down
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