linux-kernel - RE: [PATCH] clocksource/drivers/hyper-v: Re-enable VDSO_CLOCKMODE

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Date:   Wed, 12 May 2021 16:03:39 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     vkuznets <vkuznets@...hat.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
CC:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Mohammed Gamal <mgamal@...hat.com>,
        Wei Liu <wei.liu@...nel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] clocksource/drivers/hyper-v: Re-enable
 VDSO_CLOCKMODE_HVCLOCK on X86

From: Vitaly Kuznetsov <vkuznets@...hat.com> Sent: Wednesday, May 12, 2021 1:47 AM
> 
> Mohammed reports (https://bugzilla.kernel.org/show_bug.cgi?id=213029)
> the commit e4ab4658f1cf ("clocksource/drivers/hyper-v: Handle vDSO
> differences inline") broke vDSO on x86. The problem appears to be that
> VDSO_CLOCKMODE_HVCLOCK is an enum value in 'enum vdso_clock_mode' and
> '#ifdef VDSO_CLOCKMODE_HVCLOCK' branch evaluates to false (it is not
> a define). Replace it with CONFIG_X86 as it is the only arch which
> has this mode currently.
> 
> Reported-by: Mohammed Gamal <mgamal@...hat.com>
> Fixes: e4ab4658f1cf ("clocksource/drivers/hyper-v: Handle vDSO differences inline")
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  drivers/clocksource/hyperv_timer.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
> index 977fd05ac35f..e17421f5e47d 100644
> --- a/drivers/clocksource/hyperv_timer.c
> +++ b/drivers/clocksource/hyperv_timer.c
> @@ -419,7 +419,7 @@ static void resume_hv_clock_tsc(struct clocksource *arg)
>  	hv_set_register(HV_REGISTER_REFERENCE_TSC, tsc_msr);
>  }
> 
> -#ifdef VDSO_CLOCKMODE_HVCLOCK
> +#ifdef CONFIG_X86
>  static int hv_cs_enable(struct clocksource *cs)
>  {
>  	vclocks_set_used(VDSO_CLOCKMODE_HVCLOCK);
> @@ -435,7 +435,7 @@ static struct clocksource hyperv_cs_tsc = {
>  	.flags	= CLOCK_SOURCE_IS_CONTINUOUS,
>  	.suspend= suspend_hv_clock_tsc,
>  	.resume	= resume_hv_clock_tsc,
> -#ifdef VDSO_CLOCKMODE_HVCLOCK
> +#ifdef CONFIG_X86
>  	.enable = hv_cs_enable,
>  	.vdso_clock_mode = VDSO_CLOCKMODE_HVCLOCK,
>  #else
> --
> 2.31.1

Well, bummer.  I thought I was being so clever.  I hate having to base the
behavior directly on the architecture instead of the actual VDSO feature,
but I don't see a way to detect whether VDSO_CLOCKMODE_HVCLOCK is
defined or not.  The other alternative would be to add another #define
such as VDSO_CLOCKMODE_HVCLOCK_PRESENT in
arch/x86/include/asm/vdso/clocksource.h, and use it in the #ifdefs
here.  But that's a bit messy too, and I'm not sure it's worth the
trouble.  So,

Reviewed-by: Michael Kelley <mikelley@...rosoft.com>

Mohammed -- Thanks for finding this!