Re: [PATCH v2 0/3] x86/vdso: Add Hyper-V TSC page clocksource support

[Andy Lutomirski]

On Fri, Feb 17, 2017 at 2:14 AM, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
> Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes:
>
>> On Wed, 15 Feb 2017, Vitaly Kuznetsov wrote:
>>> Actually, we already have an implementation of TSC page update in KVM
>>> (see arch/x86/kvm/hyperv.c, kvm_hv_setup_tsc_page()) and the update does
>>> the following:
>>>
>>> 0) stash seq into seq_prev
>>> 1) seq = 0 making all reads from the page invalid
>>> 2) smp_wmb()
>>> 3) update tsc_scale, tsc_offset
>>> 4) smp_wmb()
>>> 5) set seq = seq_prev + 1
>>
>> I hope they handle the case where seq_prev overflows and becomes 0 :)
>>
>>> As far as I understand this helps with situations you described above as
>>> guest will notice either invalid value of 0 or seq change. In case the
>>> implementation in real Hyper-V is the same we're safe with compile
>>> barriers only.
>>
>> On x86 that's correct. smp_rmb() resolves to barrier(), but you certainly
>> need the smp_wmb() on the writer side.
>>
>> Now looking at the above your reader side code is bogus:
>>
>> +       while (1) {
>> +               sequence = tsc_pg->tsc_sequence;
>> +               if (!sequence)
>> +                       break;
>>
>> Why would you break out of the loop when seq is 0? The 0 is just telling
>> you that there is an update in progress.
>
> Not only. As far as I understand (and I *think* K. Y. pointed this out)
> when VM is migrating to another host TSC page clocksource is disabled for
> extended period of time so we're better off reading from MSR than
> looping here. With regards to VDSO this means reverting to doing normal
> syscall.

That's a crappy design.  The guest really ought to be able to
distinguish "busy, try again" from "bail and use MSR".

Thomas, I can see valid reasons why the hypervisor might want to
temporarily disable shared page-based timing, but I think it's silly
that it's conflated with indicating "retry".

But if this is indeed the ABI, we're stuck with it.