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

Makefile

@@ -141,6 +141,7 @@ ifneq (,$(filter FreeBSD OpenBSD NetBSD DragonFly,$(UNAME_S)))
   BSDHOST := 1
 endif
 
+SMP ?= 2
 QEMU    = qemu-system-x86_64
 
 # Hardware virtualisation, when the host has it.
@@ -610,7 +611,7 @@ schos.iso: $(BUILD)/kernel/kernel $(BUILD)/uefi/BOOTX64.EFI sysroot-install
 qemu-iso: schos.iso
 	$(QEMU) -serial mon:stdio $(QEMU_DISPLAY) $(QEMU_ACCEL) $(QEMU_NET) \
 	    -cdrom schos.iso -boot d \
-	    -m 1G -smp 2 -machine pc-i440fx-9.2
+	    -m 1G -smp $(SMP) -machine pc-i440fx-9.2
 
 # -----------------------------------------------------------------------
 # Kernel binary
@@ -1033,11 +1034,11 @@ OVMF_DRIVES = -drive if=pflash,format=raw,unit=0,readonly=on,file=$(OVMF_CODE) \
 # host has no /dev/kvm rather than to quietly get a slow machine.
 qemu-kvm: esp $(BUILD)/schfs.img $(OVMF_CODE) $(OVMF_VARS)
 	$(QEMU) -enable-kvm -serial mon:stdio $(QEMU_DISPLAY) $(QEMU_INPUT) $(QEMU_NET) -cpu host \
-	    $(OVMF_DRIVES) -m 1G -smp 2 -machine pc-i440fx-9.2
+	    $(OVMF_DRIVES) -m 1G -smp $(SMP) -machine pc-i440fx-9.2
 
 qemu: esp $(BUILD)/schfs.img $(OVMF_CODE) $(OVMF_VARS)
 	$(QEMU) -serial mon:stdio $(QEMU_DISPLAY) $(QEMU_INPUT) $(QEMU_ACCEL) $(QEMU_NET) \
-	    $(OVMF_DRIVES) -m 1G -smp 2 -machine pc-i440fx-9.2
+	    $(OVMF_DRIVES) -m 1G -smp $(SMP) -machine pc-i440fx-9.2
 
 # The root filesystem. Built from the staged sysroot; the kernel mounts it as
 # / in forkret().
@@ -1057,7 +1058,7 @@ $(BUILD)/schfs.img: $(MKSCHFS) $(BUILD)/sysroot.stamp
 # Once up:  cat /proc/schfs
 qemu-schfs: esp $(BUILD)/schfs.img $(OVMF_CODE) $(OVMF_VARS)
 	$(QEMU) -serial mon:stdio $(QEMU_DISPLAY) $(QEMU_ACCEL) $(QEMU_NET) \
-	    $(OVMF_DRIVES) -m 1G -smp 2 -machine pc-i440fx-9.2
+	    $(OVMF_DRIVES) -m 1G -smp $(SMP) -machine pc-i440fx-9.2
 
 # -----------------------------------------------------------------------
 # Misc