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

Andy Lutomirski
2017-02-09T22:55:50Z

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

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