tty: the switch chord is a setting, because only the host knows the answer
Third guess at a chord, and the third one would have been wrong too: a display
manager that grabs Ctrl+Alt+F<n> matches loosely, so Ctrl+Alt+Shift+F2 never
reaches the guest either. Extra modifiers do not get past a loose grab.
The real problem is that this is not knowable from inside. A key the host
swallowed is indistinguishable from a key nobody pressed, so the guest cannot
probe for a free combination, and there is no combination that is free
everywhere:
the HOST takes Alt+F4, Alt+F2, Ctrl+Alt+F1..F6 (loosely matched),
Ctrl+Alt+digit (QEMU's own console tabs)
APPLICATIONS want Alt+anything (Meta -- zsh binds Meta-digit as
digit-argument), Ctrl+letter, Ctrl+Shift+digit
(Ctrl+Shift+2 types ^@), bare F-keys
the LAYOUT wants AltGr+letter
So it is a setting. VT_SETCHORD/VT_GETCHORD, and in userspace:
chvt -c ctrl+shift+F Ctrl+Shift+F1..F6 (the new default)
chvt -c ctrl+alt+shift+F
chvt -c alt+1
chvt -c off no chord; chvt N is then the only way
chvt report the active terminal and the chord
`off` is a legitimate setting rather than a failure mode. On a machine whose
host grabs everything worth binding, `chvt N` is the honest answer.
The default moved to Ctrl+Shift+F1..F6: display managers bind Ctrl+Alt+F<n>,
not Ctrl+Shift+F<n>, and Ctrl+Shift with a *function* key is not a character
on any layout -- unlike Ctrl+Shift+digit, which is.
Caught while writing it: VTC_RALT was 0x08 while kbd.h's RALT is (1<<7), since
(1<<3) is CAPSLOCK. A chord set from userspace with AltGr would simply never
have matched, and the only symptom would have been a key that does nothing.
A _Static_assert now holds the two headers together.
Verified: Ctrl+Alt+F2 leaves the screen byte-identical, Ctrl+Shift+F2 switches,
Ctrl+Shift+F1 returns to a pixel-identical tty1, and chvt -c round-trips
through alt+1 and off and back. schfs-check passes.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2
This commit is contained in:
Nexoniarz
parent
fe2f5e1bf8
commit
7c0f7c7d01
8 changed files with 256 additions and 60 deletions
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@ -44,6 +44,8 @@ void cons_flush_input(int minor);
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int cons_fg_pgrp(int minor);
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void cons_set_fg_pgrp(int minor, int pg);
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int console_vt_hotkey(int key);
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extern uint vt_chord_mods; // kernel/dev/kbd.c -- the switch chord
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extern uint vt_chord_base;
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struct vt *vt_for_minor(int minor);
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extern uint lapic_timer_hz; // measured, kernel/arch/x86_64/lapic.c
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extern uint64 tsc_hz;
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@ -36,40 +36,43 @@
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#define KEY_INS 0xE8
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#define KEY_DEL 0xE9
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// Ctrl+Alt+Shift+F1..F6 -- switch virtual terminal.
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// Switch virtual terminal.
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//
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// Above 0xFF deliberately. kbdgetc returns an int and consoleintr takes one,
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// and every value that fits in a byte is already either a character or a
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// UTF-8 continuation byte. A chord that collided with one would be typed into
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// the terminal instead of switching away from it.
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//
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// Choosing this took two wrong answers. Anything the kernel swallows is taken
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// from every program that will ever run here, permanently, and the obvious
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// candidates are all spoken for on one side or the other:
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// *Which* chord is a runtime setting, not a constant here, and that is the
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// interesting part. Three separate parties have a claim on every candidate:
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//
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// HOST takes it, before the guest ever sees it
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// Alt+F4 the window manager closes the QEMU window
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// Alt+F2 GNOME and KDE open a run dialog
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// Ctrl+Alt+F1..F6 the display manager switches its *own* terminals
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// the HOST, which sees the keystroke first and may never pass it on
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// Alt+F4, Alt+F2 window manager
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// Ctrl+Alt+F1..F6 display manager, and it matches loosely --
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// adding Shift does not get past it
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// Ctrl+Alt+<digit> QEMU's own console tabs
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//
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// APPLICATIONS want it, and would never get it again
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// Alt+<anything> Meta. zsh's own line editor binds Meta-digit as
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// digit-argument; every readline editor does too
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// Ctrl+<letter> control characters
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// Ctrl+Shift+<digit> types a character -- Ctrl+Shift+2 is ^@
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// F1..F6 alone help, and whatever a full-screen program wants
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// APPLICATIONS, which lose the key permanently if the kernel eats it
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// Alt+<anything> Meta. zsh binds Meta-digit as digit-argument
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// Ctrl+<letter> control characters
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// Ctrl+Shift+<digit> types a character -- Ctrl+Shift+2 is ^@
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// F1..F6 alone help, and full-screen programs
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//
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// THE KEYBOARD LAYOUT wants it
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// AltGr+<letter> Polish diacritics, and on some setups AltGr is
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// delivered as Ctrl+Alt
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// the KEYBOARD LAYOUT
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// AltGr+<letter> Polish diacritics, and on some setups AltGr
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// arrives as Ctrl+Alt
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//
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// Ctrl+Alt+Shift+F1..F6 is in none of those sets. Display managers bind the
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// same chord *without* Shift, so adding it steps out of the host's way, and
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// no terminal application binds a triple-modified function key. Four keys is
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// more than one wants to press, which is correct for something done rarely
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// and deliberately -- and `chvt N` is there for everything else.
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// There is no combination that is free for everyone on every host, and the
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// guest cannot find out what its host grabs -- the keystroke simply never
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// arrives, which is indistinguishable from a chord nobody pressed. So the
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// kernel ships a default and lets it be changed: VT_SETCHORD, and `chvt
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// --chord` in userspace. Setting it to no modifiers disables the chord
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// entirely and leaves `chvt N` as the only way to switch, which is a
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// legitimate choice on a machine whose host takes everything.
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#define KEY_VT_SWITCH 0x120
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#define SCAN_F1 0x3B // F1..F6 are scancodes 0x3B..0x40
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#define SCAN_1 0x02 // 1..6 are scancodes 0x02..0x07
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// C('A') == Control-A
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#define C(x) (x - '@')
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@ -140,6 +140,32 @@ struct rlimit {
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#define VT_PROCESS 0x01
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#define VT_ACKACQ 0x02
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// VT_SETCHORD / VT_GETCHORD -- which key combination the kernel switches on.
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// SchOS's own, not Linux's: Linux hardcodes Ctrl+Alt+F<n> and nobody has ever
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// needed to move it, because Linux is the host. A guest does need to move it,
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// because the right answer depends entirely on what the host grabs first --
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// and that is not something the guest can discover.
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#define VT_GETCHORD 0x5620
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#define VT_SETCHORD 0x5621
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// Modifier bits, matched exactly. These mirror kernel/dev/kbd.h's values so
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// that userspace and the keyboard driver cannot drift apart.
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#define VTC_SHIFT 0x01 // kbd.h SHIFT (1<<0)
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#define VTC_CTRL 0x02 // kbd.h CTL (1<<1)
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#define VTC_ALT 0x04 // kbd.h ALT (1<<2)
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#define VTC_RALT 0x80 // kbd.h RALT (1<<7) -- NOT (1<<3); that is
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// CAPSLOCK, and bits 3..6 are the other locks
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// and the E0 prefix flag
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// Well-known base scancodes for the `base` field below.
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#define VTC_BASE_F1 0x3B // F1..F6
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#define VTC_BASE_1 0x02 // the number row, 1..6
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struct vt_chord {
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uint16 mods; // exactly these modifiers held, and no others
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uint16 base; // scancode of the key meaning terminal 1
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};
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struct vt_mode {
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char mode; // VT_AUTO or VT_PROCESS
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char waitv; // unused here
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