Re: [PATCH v1] kvm/x86: Hyper-V tsc page setup

[Andrey Smetanin <asmetanin@virtuozzo.com>]

On 01/22/2016 02:02 PM, Paolo Bonzini wrote:
>
>
> On 22/01/2016 11:15, Andrey Smetanin wrote:
>>>
>>> - unless KVM can use a master clock, it is incorrect to set up the TSC
>>> page this way; the sequence needs to be 0xFFFFFFFF in that case
>> 0xFFFFFFFF is not an invalid value for tsc page,
>> see https://lkml.org/lkml/2015/11/2/655
>
> oh, I see now.
>
>>> - writing the TSC page must be done while all VCPUs are stopped, because
>>> the TSC page doesn't provide the possibility for the guest to retry in
>>> the middle of an update (like seqcount in Linux doess)
>> I think Windows guest gives tsc page address at boot time and protects
>> against other vcpu's tsc page access.
>
> Sometimes the TSC is detected to be unstable and Linux switches to
> another clocksource.  At least in that case you can get a write to the
> TSC page while the guest is running.

I can't understand how write is possible.
Linux Hyper-V driver hv_vmbus.ko does the following inside hv_init() 
drivers/hv/hv.c(line 256):

                 wrmsrl(HV_X64_MSR_REFERENCE_TSC, tsc_msr.as_uint64);
                 clocksource_register_hz(&hyperv_cs_tsc, NSEC_PER_SEC/100);

So page is setup only once before registration clock source.
>
> In that case it would be enough to write a zero to tsc_sequence, which
> _can_ be done atomically while the guest is running.  However, KVM
> already has a mechanism to stop all VCPUs (KVM_REQ_MASTERCLOCK_UPDATE)
> so we might as well use it.
>
> Paolo
>