linux-kernel - RE: [PATCH v2 2/4] drivers/clocksource/hyper-v: Introduce TSC PFN getter

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Message-ID: <BYAPR21MB1688D503DF583914F4EC0356D7399@BYAPR21MB1688.namprd21.prod.outlook.com>
Date:   Wed, 2 Nov 2022 23:30:15 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>
CC:     Stanislav Kinsburskiy <stanislav.kinsburskiy@...il.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 2/4] drivers/clocksource/hyper-v: Introduce TSC PFN
 getter

From: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com> Sent: Wednesday, November 2, 2022 3:07 PM
> 
> And rework the code to use it instead of the physical address, which isn't
> required by itself.
> 
> This is a cleanup and precursor patch for upcoming support for TSC page
> mapping into Microsoft Hypervisor root partition, where TSC PFN will be
> defined by the hypervisor and not by the kernel.
> 
> Signed-off-by: Stanislav Kinsburskiy <stanislav.kinsburskiy@...il.com>
> CC: "K. Y. Srinivasan" <kys@...rosoft.com>
> CC: Haiyang Zhang <haiyangz@...rosoft.com>
> CC: Wei Liu <wei.liu@...nel.org>
> CC: Dexuan Cui <decui@...rosoft.com>
> CC: Daniel Lezcano <daniel.lezcano@...aro.org>
> CC: Thomas Gleixner <tglx@...utronix.de>
> CC: linux-hyperv@...r.kernel.org
> CC: linux-kernel@...r.kernel.org
> ---
>  drivers/clocksource/hyperv_timer.c |   14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
> index c4dbf40a3d3e..d447bc99a399 100644
> --- a/drivers/clocksource/hyperv_timer.c
> +++ b/drivers/clocksource/hyperv_timer.c
> @@ -367,6 +367,12 @@ static union {
>  } tsc_pg __aligned(PAGE_SIZE);
> 
>  static struct ms_hyperv_tsc_page *tsc_page = &tsc_pg.page;
> +static unsigned long tsc_pfn;
> +
> +static unsigned long hv_get_tsc_pfn(void)
> +{
> +	return tsc_pfn;
> +}
> 
>  struct ms_hyperv_tsc_page *hv_get_tsc_page(void)
>  {
> @@ -408,13 +414,12 @@ static void suspend_hv_clock_tsc(struct clocksource *arg)
> 
>  static void resume_hv_clock_tsc(struct clocksource *arg)
>  {
> -	phys_addr_t phys_addr = virt_to_phys(tsc_page);
>  	union hv_reference_tsc_msr tsc_msr;
> 
>  	/* Re-enable the TSC page */
>  	tsc_msr.as_uint64 = hv_get_register(HV_REGISTER_REFERENCE_TSC);
>  	tsc_msr.enable = 1;
> -	tsc_msr.pfn = __phys_to_pfn(phys_addr);
> +	tsc_msr.pfn = tsc_pfn;
>  	hv_set_register(HV_REGISTER_REFERENCE_TSC, tsc_msr.as_uint64);
>  }
> 
> @@ -498,7 +503,6 @@ static __always_inline void hv_setup_sched_clock(void
> *sched_clock) {}
>  static bool __init hv_init_tsc_clocksource(void)
>  {
>  	union hv_reference_tsc_msr tsc_msr;
> -	phys_addr_t	phys_addr;
> 
>  	if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
>  		return false;
> @@ -523,7 +527,7 @@ static bool __init hv_init_tsc_clocksource(void)
>  	}
> 
>  	hv_read_reference_counter = read_hv_clock_tsc;
> -	phys_addr = virt_to_phys(hv_get_tsc_page());
> +	tsc_pfn = __phys_to_pfn(virt_to_phys(tsc_page));

I probably wasn't specific enough when I said "your choice" in my previous
comments.   This sets tsc_pfn to a guest PFN.  In a guest with page size
other than 4096, it will be wrong when stored into tsc_msr.pfn a few
lines below.  hyperv_timer.c is currently architecture and page size
independent, and it needs to stay that way.

>  	/*
>  	 * The Hyper-V TLFS specifies to preserve the value of reserved
> @@ -534,7 +538,7 @@ static bool __init hv_init_tsc_clocksource(void)
>  	 */
>  	tsc_msr.as_uint64 = hv_get_register(HV_REGISTER_REFERENCE_TSC);
>  	tsc_msr.enable = 1;
> -	tsc_msr.pfn = __phys_to_pfn(phys_addr);
> +	tsc_msr.pfn = tsc_pfn;
>  	hv_set_register(HV_REGISTER_REFERENCE_TSC, tsc_msr.as_uint64);
> 
>  	clocksource_register_hz(&hyperv_cs_tsc, NSEC_PER_SEC/100);
>