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

include/param.h

@@ -1,5 +1,27 @@
 #pragma once
 
+// The timer interrupt rate the kernel is built around. Everything that turns
+// ticks into a duration divides by this. It used to be the literal 100 written
+// out at each site, which made it look like a fact about the hardware; it is
+// not. It is a target, and kernel/arch/x86_64/lapic.c calibrates the LAPIC
+// timer against the PIT to actually hit it.
+#define HZ           100  // timer interrupts per second
+
+// The tick rate the *user-space ABI* is defined in, which is a different thing
+// from HZ and must not follow it. glibc and musl both answer sysconf(_SC_CLK_TCK)
+// with a hardcoded 100, so times() and /proc/stat are read as hundredths of a
+// second no matter what the kernel's internal rate is. Linux calls this USER_HZ
+// and scales at the syscall boundary; so does SchOS, in the two places below.
+#define USER_HZ      100
+
+// Converting a POSIX timespec/timeval into ticks. These used to appear as the
+// bare literals 10000000 and 10000 at each call site, where they looked like
+// magic numbers rather than what they are: the length of one tick, expressed
+// in the units the caller happens to be using.
+#define NSEC_PER_TICK (1000000000 / HZ)
+#define USEC_PER_TICK (1000000 / HZ)
+#define MSEC_PER_TICK (1000 / HZ)
+
 #define NPROC        128  // maximum number of processes
 #define KSTACKSIZE  4096  // size of per-process kernel stack (increased for x64)
 #define KSTACK_CANARY 0xDEADC0DEDEADC0DEULL  // sentinel at kstack[0], see kstack_check_canary()