linux-kernel - RE: [PATCH 2/2] x86/vdso: Add VCLOCK

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Message-ID: <DM5PR03MB24903D71121176FFAED30E33A0450@DM5PR03MB2490.namprd03.prod.outlook.com>
Date:   Thu, 9 Feb 2017 20:45:52 +0000
From:   KY Srinivasan <kys@...rosoft.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
CC:     "x86@...nel.org" <x86@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Haiyang Zhang" <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>
Subject: RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method



> -----Original Message-----
> From: Thomas Gleixner [mailto:tglx@...utronix.de]
> Sent: Thursday, February 9, 2017 9:08 AM
> To: Vitaly Kuznetsov <vkuznets@...hat.com>
> Cc: x86@...nel.org; Andy Lutomirski <luto@...capital.net>; Ingo Molnar
> <mingo@...hat.com>; H. Peter Anvin <hpa@...or.com>; KY Srinivasan
> <kys@...rosoft.com>; Haiyang Zhang <haiyangz@...rosoft.com>; Stephen
> Hemminger <sthemmin@...rosoft.com>; Dexuan Cui
> <decui@...rosoft.com>; linux-kernel@...r.kernel.org;
> devel@...uxdriverproject.org; virtualization@...ts.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