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

Andy Lutomirski
Thu, 09 Feb 2017 22:55:50 -0000

On Thu, Feb 9, 2017 at 12:45 PM, KY Srinivasan <kys at microsoft.com>
wrote:
>
>
>> -----Original Message-----
>> From: Thomas Gleixner [mailto:tglx at linutronix.de]
>> Sent: Thursday, February 9, 2017 9:08 AM
>> To: Vitaly Kuznetsov <vkuznets at redhat.com>
>> Cc: x86 at kernel.org; Andy Lutomirski <luto at amacapital.net>;
Ingo Molnar
>> <mingo at redhat.com>; H. Peter Anvin <hpa at zytor.com>;
KY Srinivasan
>> <kys at microsoft.com>; Haiyang Zhang <haiyangz at
microsoft.com>; Stephen
>> Hemminger <sthemmin at microsoft.com>; Dexuan Cui
>> <decui at microsoft.com>; linux-kernel at vger.kernel.org;
>> devel at linuxdriverproject.org; virtualization at
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.

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