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

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Date:   Thu, 9 Feb 2017 14:55:50 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     KY Srinivasan <kys@...rosoft.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>, 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

On Thu, Feb 9, 2017 at 12:45 PM, KY Srinivasan <kys@...rosoft.com> wrote:
>
>
>> -----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.

So what's the actual protocol?  When the hypervisor updates the page,
does it freeze all guest cpus?  If not, how does it maintain
atomicity?

--Andy