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RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method

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> -----Original Message-----
> From: Thomas Gleixner [mailto:tglx@linutronix.de]
> Sent: Thursday, February 9, 2017 9:08 AM
> To: Vitaly Kuznetsov <vkuznets@redhat.com>
> Cc: x86@kernel.org; Andy Lutomirski <luto@amacapital.net>; Ingo Molnar
> <mingo@redhat.com>; H. Peter Anvin <hpa@zytor.com>; KY Srinivasan
> <kys@microsoft.com>; Haiyang Zhang <haiyangz@microsoft.com>; Stephen
> Hemminger <sthemmin@microsoft.com>; Dexuan Cui
> <decui@microsoft.com>; linux-kernel@vger.kernel.org;
> devel@linuxdriverproject.org; virtualization@lists.linux-foundation.org
> Subject: Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read
> method
> 
> On Thu, 9 Feb 2017, Vitaly Kuznetsov wrote:
> > +#ifdef CONFIG_HYPERV_TSCPAGE
> > +static notrace u64 vread_hvclock(int *mode)
> > +{
> > +	const struct ms_hyperv_tsc_page *tsc_pg =
> > +		(const struct ms_hyperv_tsc_page *)&hvclock_page;
> > +	u64 sequence, scale, offset, current_tick, cur_tsc;
> > +
> > +	while (1) {
> > +		sequence = READ_ONCE(tsc_pg->tsc_sequence);
> > +		if (!sequence)
> > +			break;
> > +
> > +		scale = READ_ONCE(tsc_pg->tsc_scale);
> > +		offset = READ_ONCE(tsc_pg->tsc_offset);
> > +		rdtscll(cur_tsc);
> > +
> > +		current_tick = mul_u64_u64_shr(cur_tsc, scale, 64) + offset;
> > +
> > +		if (READ_ONCE(tsc_pg->tsc_sequence) == sequence)
> > +			return current_tick;
> 
> That sequence stuff lacks still a sensible explanation. It's fundamentally
> different from the sequence counting we do in the kernel, so documentation
> for it is really required.

The host is updating multiple fields in this shared TSC page and the sequence number is
used to ensure that the guest sees a consistent set values published. If I remember
correctly, Xen has a similar mechanism.

K. Y
> 
> Thanks,
> 
> 	tglx

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[PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-02-09 15:20 +0100
  Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read  method Thomas Gleixner <tglx@linutronix.de> - 2017-02-09 18:10 +0100
    RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method Stephen Hemminger <sthemmin@microsoft.com> - 2017-02-09 19:30 +0100
      Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read  method Thomas Gleixner <tglx@linutronix.de> - 2017-02-10 15:10 +0100
        RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method Stephen Hemminger <sthemmin@microsoft.com> - 2017-02-10 17:40 +0100
          RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read  method Thomas Gleixner <tglx@linutronix.de> - 2017-02-10 19:30 +0100
            RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method Dexuan Cui <decui@microsoft.com> - 2017-02-13 09:00 +0100
              RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read  method Thomas Gleixner <tglx@linutronix.de> - 2017-02-13 10:30 +0100
      Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-02-10 15:50 +0100
    RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method KY Srinivasan <kys@microsoft.com> - 2017-02-09 21:50 +0100
      Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method Andy Lutomirski <luto@amacapital.net> - 2017-02-10 00:00 +0100
        Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method Stephen Hemminger <stephen@networkplumber.org> - 2017-02-10 00:20 +0100
        Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-02-10 13:20 +0100
    Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-02-10 12:10 +0100
      Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read  method Thomas Gleixner <tglx@linutronix.de> - 2017-02-10 12:40 +0100

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