kernel: the clock ran 7x slow because the GPU never acknowledged its interrupt

fastfetch reported 3 seconds of uptime after 15 real ones, `time` claimed
3.12s for a 20-second command, and ping's one-second interval was closer to
six. One cause, not three: `ticks` was advancing at ~14.5 Hz instead of 100.

It was not the timer. virtio-gpu-pci shares IRQ 11 with the e1000 and
registered no interrupt handler at all. PCI INTx is level-triggered, so the
device held the line asserted, the kernel signalled EOI, and it fired again
immediately -- 72,000 interrupts a second, 83% of CPU0, starving the tick.
The e1000's handler read ICR as zero half a million times, correctly
answering "not mine", which is what made it look like a network-free problem.

Reading virtio's ISR byte clears it and drops the line. That is the fix.

irq_dispatch now measures each line's rate against the TSC -- not ticks,
which is the first casualty of a storm -- and names the attached handlers
once above 20000/s. It reproduces the original fault as a single line.

Found along the way, and fixed because they were real:

  - lapicinit loaded TICR with xv6's uncalibrated 10000000, whose own comment
    admitted it was never measured. calibrate_timers() now measures the LAPIC
    and the TSC against the 8254 PIT. (It came back at 999.9 MHz, so this was
    not the bug -- but the next machine is not obliged to be 1 GHz.)
  - microdelay() was an empty function body. Every microdelay(1000) in the
    xHCI reset path and the USB hub power-on sequence returned instantly.
  - HZ is a constant in param.h now, not the literal 100 written out at a
    dozen ticks-to-seconds conversions. USER_HZ is separate and deliberately
    does not follow it: musl answers sysconf(_SC_CLK_TCK) with 100 always, so
    times() and /proc/stat scale at the syscall boundary the way Linux does.
  - TIOCGWINSZ answered a hardcoded 24x80. gfx_cols()/gfx_rows() have known
    the real geometry all along; at 1024x768 the console is 128x48, so every
    program that lays out a screen was formatting for the wrong one.
  - TCFLSH fell through to the catch-all `return 0`, claiming to discard
    input without discarding any.

uptime now advances 5.01 seconds per 5 real seconds. schfs-check passes.

Full investigation, including the two wrong hypotheses and the measurement
flaw that protected them, in docs/history/kernel.md section 13.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2

kernel/sys/sys_time.c

@@ -16,7 +16,7 @@ sys_alarm(void)
     int seconds;
     if (argint(0, &seconds) < 0)
         return -1;
-    myproc()->alarm_ticks = seconds * 100;
+    myproc()->alarm_ticks = seconds * HZ;
     return 0;
 }
 
@@ -48,7 +48,7 @@ sys_nanosleep(void)
     if (sec > 0) {
         acquire(&tickslock);
         uint t0 = ticks;
-        while (ticks - t0 < (uint)(sec * 100)) {
+        while (ticks - t0 < (uint)(sec * HZ)) {
             if (myproc()->killed) { release(&tickslock); return -EINTR; }
             sleep(&ticks, &tickslock);
         }
@@ -89,8 +89,8 @@ sys_setitimer(void)
     if (which == 0 && newp) {
         struct itimerval iv;
         if (copy_from_user(&iv, newp, sizeof(iv)) == 0)
-            myproc()->alarm_ticks = (int)(iv.it_value.tv_sec * 100 +
-                                      iv.it_value.tv_usec / 10000);
+            myproc()->alarm_ticks = (int)(iv.it_value.tv_sec * HZ +
+                                      iv.it_value.tv_usec / USEC_PER_TICK);
     }
     return 0;
 }
@@ -129,7 +129,7 @@ sys_clock_nanosleep(void)
     if (sec > 0) {
         acquire(&tickslock);
         uint t0 = ticks;
-        while (ticks - t0 < (uint)(sec * 100)) {
+        while (ticks - t0 < (uint)(sec * HZ)) {
             if (myproc()->killed) { release(&tickslock); return -EINTR; }
             sleep(&ticks, &tickslock);
         }