Re: [PATCH 1/2] KVM: x86: implement KVM_SET_TSC_PRECISE/KVM_GET_TSC_PRECISE

[Paolo Bonzini]

On 30/11/20 16:58, Maxim Levitsky wrote:
> > This is mostly useful for userspace that doesn't disable the quirk, right?
> Isn't this the opposite? If I understand the original proposal correctly,
> the reason that we include the TSC_ADJUST in the new ioctl, is that
> we would like to disable the special kvm behavior (that is disable the quirk),
> which would mean that tsc will jump on regular host initiated TSC_ADJUST write.
>
> To avoid this, userspace would set TSC_ADJUST through this new interface.

Yeah, that makes sense.  It removes the need to think "I have to set TSC 
adjust before TSC".

> Do you think that this is an issue? If so I can make the code work with
> signed numbers.

Not sure if it's an issue, but I prefer to make the API "less 
surprising" for userspace.  Who knows how it will be used.

> About nsec == 0, this is to allow to use this API for VM initialization.
> (That is to call KVM_SET_TSC_PRECISE prior to doing KVM_GET_TSC_PRECISE)

I prefer using flags for that purpose.

Paolo