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

Vitaly Kuznetsov
2017-02-10T12:15:00Z

Andy Lutomirski <luto@amacapital.net> writes:

> 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?

I don't really know how it is implemented server-side but I *think* that
freezing all CPUs is only required when we want to update *both*
ReferenceTscScale and ReferenceTscOffset at the same time (as Hyper-V is
64-bit only so it can always atomically update 64-bit values)...

--
Vitaly