RE: [PATCH 1/2] clocksource/Hyper-v: Allocate Hyper-V tsc page statically

[Michael Kelley <mikelley@microsoft.com>]

From: lantianyu1986@gmail.com <lantianyu1986@gmail.com> Sent: Monday, July 29, 2019 12:53 AM
> 
> This is to prepare to add Hyper-V sched clock callback and move
> Hyper-V reference TSC initialization much earlier in the boot
> process when timestamp is 0. So no discontinuity is observed
> when pv_ops.time.sched_clock to calculate its offset. This earlier
> initialization requires that the Hyper-V TSC page be allocated
> statically instead of with vmalloc(), so fixup the references
> to the TSC page and the method of getting its physical address.

I'd suggest tweaking the commit message wording a bit:

Prepare to add Hyper-V sched clock callback and move Hyper-V
Reference TSC initialization much earlier in the boot process.  Earlier
initialization is needed so that it happens while the timestamp value
is still 0 and no discontinuity in the timestamp will occur when 
pv_ops.time.sched_clock calculates its offset.  The earlier
initialization requires that the Hyper-V TSC page be allocated
statically instead of with vmalloc(), so fixup the references
to the TSC page and the method of getting its physical address.

> 
> Signed-off-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
> ---
>  arch/x86/entry/vdso/vma.c          |  2 +-
>  drivers/clocksource/hyperv_timer.c | 12 ++++--------
>  2 files changed, 5 insertions(+), 9 deletions(-)
> 
> @@ -280,12 +280,8 @@ static bool __init hv_init_tsc_clocksource(void)
>  	if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
>  		return false;
> 
> -	tsc_pg = vmalloc(PAGE_SIZE);
> -	if (!tsc_pg)
> -		return false;
> -
>  	hyperv_cs = &hyperv_cs_tsc;
> -	phys_addr = page_to_phys(vmalloc_to_page(tsc_pg));
> +	phys_addr = virt_to_phys(&tsc_pg) & PAGE_MASK;

The and'ing with PAGE_MASK isn't needed.  You've set up tsc_pg
to ensure it is page aligned, so there's no need to mask out any
low order bits.  That's why the previous code didn't do the masking
either.

> 
>  	/*
>  	 * The Hyper-V TLFS specifies to preserve the value of reserved
> --
> 2.14.5