Re: [PATCH v3 1/1] i386: Make pmtimer tsc calibration not take 51 seconds to fail

[Daniel Kiper]

On Thu, Jul 14, 2022 at 05:42:51PM -0400, Robbie Harwood wrote:
> From: Peter Jones <pjones@redhat.com>
>
> 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 compiled with
> GRUB_PMTIMER_IGNORE_BAD_READS defined (so as not to trip the bad read
> test) 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://firmware.intel.com/sites/default/files/UEFIDevKit_S1200RP_vB8.zip
>
> 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>
> ---
>  grub-core/kern/i386/tsc_pmtimer.c | 112 ++++++++++++++++++++++++------
>  1 file changed, 92 insertions(+), 20 deletions(-)
>
> diff --git a/grub-core/kern/i386/tsc_pmtimer.c 
> b/grub-core/kern/i386/tsc_pmtimer.c
> index c9c3616997..d9b3765b01 100644
> --- a/grub-core/kern/i386/tsc_pmtimer.c
> +++ b/grub-core/kern/i386/tsc_pmtimer.c
> @@ -28,40 +28,104 @@
>  #include <grub/acpi.h>
>  #include <grub/cpu/io.h>
>
> +/*
> + * Define GRUB_PMTIMER_IGNORE_BAD_READS if you're trying to test a timer 
> that's
> + * present but doesn't keep time well.
> + */
> +// #define GRUB_PMTIMER_IGNORE_BAD_READS
> +
>  grub_uint64_t
>  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;
> +  unsigned int num_iter = 0;
> +#ifndef GRUB_PMTIMER_IGNORE_BAD_READS
> +  int bad_reads = 0;
> +#endif
>
> -  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();
> +
> +#ifndef GRUB_PMTIMER_IGNORE_BAD_READS

I still do not understand why we need compile this code conditionally.
Why cannot we build it always and enable only when debug is set to
pmtimer/all? Or use another variable to enable this test?

> +      /*
> +       * 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);
> +       grub_dprintf ("pmtimer", "timer is broken; giving up.\n");

It seems to me this grub_dprintf() should be moved...

> +
> +       if (bad_reads == 10)

... here.

> +         return 0;
> +     }
> +#endif
> +
> +      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) ||

s/grub_uint32_t/unsigned int/? And please add space between ")" and 
num_pm_ticks.

> +       end_tsc - start_tsc > 40000000)

You can put this in one line with "if" above. In general I am OK with
lines a bit longer than 80 chars...

> +     {
> +       grub_dprintf ("pmtimer",
> +                     "pmtimer delta is 0x%"PRIxGRUB_UINT64_T" (%u 
> iterations)\n",
> +                     cur - start, num_iter);
> +       grub_dprintf ("pmtimer",
> +                     "tsc delta is implausible: 0x%"PRIxGRUB_UINT64_T"\n",
> +                     end_tsc - start_tsc);
> +       return 0;
> +     }
>      }
>  }
>
> @@ -74,12 +138,20 @@ 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.  */
> +  /*
> +   * It's 3.579545 MHz clock. Wait 1 ms.
> +   */

Please drop this change.

Daniel