Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE

[Paolo Bonzini]

On 10/12/20 18:59, Oliver Upton wrote:
> However, I don't believe we can assume the guest's TSCs to be synchronized,
> even if sane guests will never touch them. In this case, I think a per-vCPU
> ioctl is still warranted, allowing userspace to get at the guest CPU adjust
> component of Thomas' equation below (paraphrased):
>
>          TSC guest CPU = host tsc base + guest base offset + guest CPU adjust

Right now that would be:

- KVM_GET_TSC_STATE (vm) returns host tsc base + guest base offset (plus 
the associated time)

- KVM_GET_MSR *without* KVM_X86_QUIRK_TSC_HOST_ACCESS for guest CPU adjust

and the corresponding SET ioctls.  What am *I* missing?

> Alternatively, a write from userspace to the guest's IA32_TSC_ADJUST with
> KVM_X86_QUIRK_TSC_HOST_ACCESS could have the same effect, but that seems to be
> problematic for a couple reasons. First, depending on the guest's CPUID the
> TSC_ADJUST MSR may not even be available, meaning that the guest could've used
> IA32_TSC to adjust the TSC (eww).

Indeed, the host should always be able to read/write IA32_TSC and 
IA32_TSC_ADJUST.

Thanks,

Paolo

> Second, userspace replaying writes to IA32_TSC
> (in the case IA32_TSC_ADJUST doesn't exist for the guest) seems_very_
> unlikely to work given all the magic handling that KVM does for
> writes to it.
>
> Is this roughly where we are or have I entirely missed the mark?:-)