kern/i386/tsc_pmtimer: Make pmtimer tsc calibration not take 51 seconds to fail

On my laptop running at 2.4GHz, if I run a VM where tsc calibration
using pmtimer will fail presuming a broken pmtimer, it takes ~51 seconds
to do so (as measured with the stopwatch on my phone), with a tsc delta
of 0x1cd1c85300, or around 125 billion cycles.

If instead of trying to wait for 5-200ms to show up on the pmtimer, we
try to wait for 5-200us, it decides it's broken in ~0x2626aa0 TSCs, aka
~2.4 million cycles, or more or less instantly.

Additionally, this reading the pmtimer was returning 0xffffffff anyway,
and that's obviously an invalid return. I've added a check for that and
0 so we don't bother waiting for the test if what we're seeing is dead
pins with no response at all.

If "debug" includes "pmtimer", you will see one of the following three
outcomes. If pmtimer gives all 0 or all 1 bits, you will see:

  pmtimer: 0xffffff bad_reads: 1
  pmtimer: 0xffffff bad_reads: 2
  pmtimer: 0xffffff bad_reads: 3
  pmtimer: 0xffffff bad_reads: 4
  pmtimer: 0xffffff bad_reads: 5
  pmtimer: 0xffffff bad_reads: 6
  pmtimer: 0xffffff bad_reads: 7
  pmtimer: 0xffffff bad_reads: 8
  pmtimer: 0xffffff bad_reads: 9
  pmtimer: 0xffffff bad_reads: 10
  timer is broken; giving up.

This outcome was tested using qemu+kvm with UEFI (OVMF) firmware and
these options: -machine pc-q35-2.10 -cpu Broadwell-noTSX

If pmtimer gives any other bit patterns but is not actually marching
forward fast enough to use for clock calibration, you will see:

  pmtimer delta is 0x0 (1904 iterations)
  tsc delta is implausible: 0x2626aa0

This outcome was tested using GRUB patched to not ignore bad reads using
qemu+kvm with UEFI (OVMF) firmware, and these options:
-machine pc-q35-2.10 -cpu Broadwell-noTSX

If pmtimer actually works, you'll see something like:

  pmtimer delta is 0xdff
  tsc delta is 0x278756

This outcome was tested using qemu+kvm with UEFI (OVMF) firmware, and
these options: -machine pc-i440fx-2.4 -cpu Broadwell-noTSX

I've also tested this outcome on a real Intel Xeon E3-1275v3 on an Intel
Server Board S1200V3RPS using the SDV.RP.B8 "Release" build here:
https://www.intel.com/content/www/us/en/download/674448/firmware-update-for-the-intel-server-board-s1200rp-uefi-development-kit-release-vb8.html

Signed-off-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Robbie Harwood <rharwood@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

1 files changed, 78 insertions, 19 deletions

diff --git a/grub-core/kern/i386/tsc_pmtimer.c b/grub-core/kern/i386/tsc_pmtimer.c
index c9c361699..5c03c510a 100644
--- a/grub-core/kern/i386/tsc_pmtimer.c
+++ b/grub-core/kern/i386/tsc_pmtimer.c
@@ -33,35 +33,88 @@ grub_pmtimer_wait_count_tsc (grub_port_t pmtimer,
 			     grub_uint16_t num_pm_ticks)
 {
   grub_uint32_t start;
-  grub_uint32_t last;
-  grub_uint32_t cur, end;
+  grub_uint64_t cur, end;
   grub_uint64_t start_tsc;
   grub_uint64_t end_tsc;
-  int num_iter = 0;
+  grub_uint32_t num_iter = 0;
+  int bad_reads = 0;
 
-  start = grub_inl (pmtimer) & 0xffffff;
-  last = start;
+  /*
+   * Some timers are 24-bit and some are 32-bit, but it doesn't make much
+   * difference to us. Caring which one we have isn't really worth it since
+   * the low-order digits will give us enough data to calibrate TSC. So just
+   * mask the top-order byte off.
+   */
+  cur = start = grub_inl (pmtimer) & 0x00ffffffUL;
   end = start + num_pm_ticks;
   start_tsc = grub_get_tsc ();
   while (1)
     {
-      cur = grub_inl (pmtimer) & 0xffffff;
-      if (cur < last)
-	cur |= 0x1000000;
-      num_iter++;
+      cur &= 0xffffffffff000000ULL;
+
+      /* Only take the low-order 24-bit for the reason explained above. */
+      cur |= grub_inl (pmtimer) & 0x00ffffffUL;
+
+      end_tsc = grub_get_tsc();
+
+      /*
+       * If we get 10 reads in a row that are obviously dead pins, there's no
+       * reason to do this thousands of times.
+       */
+      if (cur == 0xffffffUL || cur == 0)
+	{
+	  bad_reads++;
+	  grub_dprintf ("pmtimer",
+			"pmtimer: 0x%"PRIxGRUB_UINT64_T" bad_reads: %d\n",
+			cur, bad_reads);
+
+	  if (bad_reads == 10)
+            {
+              grub_dprintf ("pmtimer", "timer is broken; giving up.\n");
+              return 0;
+            }
+	}
+
+      if (cur < start)
+	cur += 0x1000000;
+
       if (cur >= end)
 	{
-	  end_tsc = grub_get_tsc ();
+	  grub_dprintf ("pmtimer", "pmtimer delta is 0x%"PRIxGRUB_UINT64_T"\n",
+			cur - start);
+	  grub_dprintf ("pmtimer", "tsc delta is 0x%"PRIxGRUB_UINT64_T"\n",
+			end_tsc - start_tsc);
 	  return end_tsc - start_tsc;
 	}
-      /* Check for broken PM timer.
-	 50000000 TSCs is between 5 ms (10GHz) and 200 ms (250 MHz)
-	 if after this time we still don't have 1 ms on pmtimer, then
-	 pmtimer is broken.
+
+      /*
+       * Check for broken PM timer. 1ms at 10GHz should be 1E+7 TSCs; at
+       * 250MHz it should be 2.5E5. So if after 4E+7 TSCs on a 10GHz machine,
+       * we should have seen pmtimer show 4ms of change (i.e. cur =~ start + 14320);
+       * on a 250MHz machine that should be 160ms (start + 572800). If after
+       * this a time we still don't have 1ms on pmtimer, then pmtimer is broken.
+       *
+       * Likewise, if our code is perfectly efficient and introduces no delays
+       * whatsoever, on a 10GHz system we should see a TSC delta of 3580 in
+       * ~3580 iterations. On a 250MHz machine that should be ~900 iterations.
+       *
+       * With those factors in mind, there are two limits here. There's a hard
+       * limit here at 8x our desired pm timer delta. This limit was picked as
+       * an arbitrarily large value that's still not a lot of time to humans,
+       * because if we get that far this is either an implausibly fast machine
+       * or the pmtimer is not running. And there is another limit on a 4 ms TSC
+       * delta on a 10 GHz clock, without seeing cur converge on our target value.
        */
-      if ((num_iter & 0xffffff) == 0 && grub_get_tsc () - start_tsc > 5000000) {
-	return 0;
-      }
+      if ((++num_iter > (grub_uint32_t) num_pm_ticks << 3UL) || end_tsc - start_tsc > 40000000)
+	{
+	  grub_dprintf ("pmtimer",
+			"pmtimer delta is 0x%"PRIxGRUB_UINT64_T" (%"PRIxGRUB_UINT32_T" iterations)\n",
+			cur - start, num_iter);
+	  grub_dprintf ("pmtimer",
+			"tsc delta is implausible: 0x%"PRIxGRUB_UINT64_T"\n",
+			end_tsc - start_tsc);
+	  return 0;
+	}
     }
 }
 
@@ -74,10 +127,16 @@ grub_tsc_calibrate_from_pmtimer (void)
 
   fadt = grub_acpi_find_fadt ();
   if (!fadt)
-    return 0;
+    {
+      grub_dprintf ("pmtimer", "No FADT found; not using pmtimer.\n");
+      return 0;
+    }
   pmtimer = fadt->pmtimer;
   if (!pmtimer)
-    return 0;
+    {
+      grub_dprintf ("pmtimer", "FADT does not specify pmtimer; skipping.\n");
+      return 0;
+    }
 
   /* It's 3.579545 MHz clock. Wait 1 ms.  */
   tsc_diff = grub_pmtimer_wait_count_tsc (pmtimer, 3580);