Re: [PATCH 1/2] x86/hpet: Use another crystalball to evaluate HPET usability

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Oct 19, 2021 at 09:07:39AM +0200, Jan Beulich wrote:
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> 
> On recent Intel systems the HPET stops working when the system reaches PC10
> idle state.
> 
> The approach of adding PCI ids to the early quirks to disable HPET on
> these systems is a whack a mole game which makes no sense.
> 
> Check for PC10 instead and force disable HPET if supported. The check is
> overbroad as it does not take ACPI, mwait-idle enablement and command
> line parameters into account. That's fine as long as there is at least
> PMTIMER available to calibrate the TSC frequency. The decision can be
> overruled by adding "clocksource=hpet" on the Xen command line.
> 
> Remove the related PCI quirks for affected Coffee Lake systems as they
> are not longer required. That should also cover all other systems, i.e.
> Ice Lake, Tiger Lake, and newer generations, which are most likely
> affected by this as well.
> 
> Fixes: Yet another hardware trainwreck
> Reported-by: Jakub Kicinski <kuba@xxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> [Linux commit: 6e3cd95234dc1eda488f4f487c281bac8fef4d9b]
> 
> I have to admit that the purpose of checking CPUID5_ECX_INTERRUPT_BREAK
> is unclear to me, but I didn't want to diverge in technical aspects from
> the Linux commit.
> 
> In mwait_pc10_supported(), besides some cosmetic adjustments, avoid UB
> from shifting left a signed 4-bit constant by 28 bits.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Just one comment below.

> ---
> v2: Fully different replacement of "x86: avoid HPET use also on certain
>     Coffee Lake H".
> 
> --- a/xen/arch/x86/time.c
> +++ b/xen/arch/x86/time.c
> @@ -34,6 +34,7 @@
>  #include <asm/fixmap.h>
>  #include <asm/guest.h>
>  #include <asm/mc146818rtc.h>
> +#include <asm/mwait.h>
>  #include <asm/div64.h>
>  #include <asm/acpi.h>
>  #include <asm/hpet.h>
> @@ -395,14 +396,43 @@ static int64_t __init init_hpet(struct p
>              }
>  
>          /*
> -         * Some Coffee Lake platforms have a skewed HPET timer once the SoCs
> -         * entered PC10.
> +         * Some Coffee Lake and later platforms have a skewed HPET timer once
> +         * they entered PC10.
> +         *
> +         * Check whether the system supports PC10. If so force disable HPET 
> as
> +         * that stops counting in PC10. This check is overbroad as it does 
> not
> +         * take any of the following into account:
> +         *
> +         *   - ACPI tables
> +         *   - Enablement of mwait-idle
> +         *   - Command line arguments which limit mwait-idle C-state support
> +         *
> +         * That's perfectly fine. HPET is a piece of hardware designed by
> +         * committee and the only reasons why it is still in use on modern
> +         * systems is the fact that it is impossible to reliably query TSC 
> and
> +         * CPU frequency via CPUID or firmware.
> +         *
> +         * If HPET is functional it is useful for calibrating TSC, but this 
> can
> +         * be done via PMTIMER as well which seems to be the last remaining
> +         * timer on X86/INTEL platforms that has not been completely 
> wreckaged
> +         * by feature creep.
> +         *
> +         * In theory HPET support should be removed altogether, but there are
> +         * older systems out there which depend on it because TSC and APIC 
> timer
> +         * are dysfunctional in deeper C-states.
>           */
> -        if ( pci_conf_read16(PCI_SBDF(0, 0, 0, 0),
> -                             PCI_VENDOR_ID) == PCI_VENDOR_ID_INTEL &&
> -             pci_conf_read16(PCI_SBDF(0, 0, 0, 0),
> -                             PCI_DEVICE_ID) == 0x3ec4 )
> -            hpet_address = 0;
> +        if ( mwait_pc10_supported() )
> +        {
> +            uint64_t pcfg;
> +
> +            rdmsrl(MSR_PKG_CST_CONFIG_CONTROL, pcfg);
> +            if ( (pcfg & 0xf) < 8 )
> +                /* nothing */;
> +            else if ( !strcmp(opt_clocksource, pts->id) )
> +                printk("HPET use requested via command line, but 
> dysfunctional in PC10\n");
> +            else
> +                hpet_address = 0;

Should we print a message that HPET is being disabled?

Thanks, Roger.