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

[Paolo Bonzini]

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.

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