linux-kernel - Re: [PATCH] x86/hyper-v: enable TSC page clocksource on 32bit

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Date:   Wed, 21 Aug 2019 23:17:35 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
cc:     linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] x86/hyper-v: enable TSC page clocksource on 32bit

On Wed, 21 Aug 2019, Vitaly Kuznetsov wrote:

> There is no particular reason to not enable TSC page clocksource
> on 32-bit. mul_u64_u64_shr() is available and despite the increased
> computational complexity (compared to 64bit) TSC page is still a huge
> win compared to MSR-based clocksource.
> 
> In-kernel reads:
>   MSR based clocksource: 3361 cycles
>   TSC page clocksource: 49 cycles
> 
> Reads from userspace (unilizing vDSO in case of TSC page):
>   MSR based clocksource: 5664 cycles
>   TSC page clocksource: 131 cycles
> 
> Enabling TSC page on 32bits allows us to get rid of CONFIG_HYPERV_TSCPAGE

s/allows us/allows/

> as it is now not any different from CONFIG_HYPERV_TIMER.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/include/asm/vdso/gettimeofday.h |  6 +++---
>  drivers/clocksource/hyperv_timer.c       | 11 -----------
>  drivers/hv/Kconfig                       |  3 ---
>  include/clocksource/hyperv_timer.h       |  6 ++----
>  4 files changed, 5 insertions(+), 21 deletions(-)

Really nice cleanup as a side effect of adding functionality.

Thanks,

	tglx