Re: [PATCH] KVM: x86: add bit to indicate correct tsc_shift

[Paolo Bonzini <pbonzini@redhat.com>]

On 1/20/23 02:06, Marcelo Tosatti wrote:
> Before commit 78db6a5037965429c04d708281f35a6e5562d31b,
> kvm_guest_time_update() would use vcpu->virtual_tsc_khz to calculate
> tsc_shift value in the vcpus pvclock structure written to guest memory.
> 
> For those kernels, if vcpu->virtual_tsc_khz != tsc_khz (which can be the
> case when guest state is restored via migration, or if tsc-khz option is
> passed to QEMU), and TSC scaling is not enabled (which happens if the
> difference between the frequency requested via KVM_SET_TSC_KHZ and the
> host TSC KHZ is smaller than 250ppm), then there can be a difference
> between what KVM_GET_CLOCK would return and what the guest reads as
> kvmclock value.

I don't think it's justifiable to further complicate the userspace API 
for a bug that's been fixed six years ago.  I'd be very surprised if any 
combination of modern upstream {QEMU,kernel} is going to do a successful 
migration from such an old {QEMU,kernel}.  RHEL/CentOS are able to do so 
because *specific pairs* have been tested, but as far as upstream is 
concerned this adds complexity that absolutely no one will use.

I replied on the QEMU patch with further suggestions.

Paolo