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?:-)
