Re: [PATCH v4 1/1] i386: Make pmtimer tsc calibration not take 51 seconds to fail

[Daniel Kiper]

On Thu, Jul 28, 2022 at 02:34:16PM -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 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://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>

Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

Daniel