2017-12-01 14:13+0100, Vitaly Kuznetsov:
> Currently, KVM is able to work in 'masterclock' mode passing
> PVCLOCK_TSC_STABLE_BIT to guests when the clocksource we use on the host
> is TSC. When running nested on Hyper-V we normally use a different one:
> TSC page which is resistant to TSC frequency changes on event like L1
> migration. Add support for it in KVM.
>
> The only non-trivial change in the patch is in vgettsc(): when updating
> our gtod copy we now need to get both the clockread and tsc value.
>
> Signed-off-by: Vitaly Kuznetsov <vkuzn...@redhat.com>
> ---
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> @@ -1374,6 +1375,11 @@ static u64 compute_guest_tsc(struct kvm_vcpu *vcpu,
> s64 kernel_ns)
> +static inline int gtod_cs_mode_good(int mode)
"good" isn't saying much; I'd like to express that TSC is the underlying
clock ...
What about "bool gtod_is_based_on_tsc()"?
> +{
> + return mode == VCLOCK_TSC || mode == VCLOCK_HVCLOCK;
> +}
> +
> @@ -1606,9 +1625,17 @@ static inline u64 vgettsc(u64 *cycle_now)
> long v;
> struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
>
> - *cycle_now = read_tsc();
> + if (gtod->clock.vclock_mode == VCLOCK_HVCLOCK) {
> + u64 tsc_pg_val;
> +
> + tsc_pg_val = hv_read_tsc_page_tsc(hv_get_tsc_page(), cycle_now);
This function might fail to update cycle_now and return -1.
I guess we should propagate the failure in that case.
> + v = (tsc_pg_val - gtod->clock.cycle_last) & gtod->clock.mask;
> + } else {
> + /* VCLOCK_TSC */
> + *cycle_now = read_tsc();
> + v = (*cycle_now - gtod->clock.cycle_last) & gtod->clock.mask;
cycle_now is getting pretty confusing -- it still is TSC timestamp, but
now we also have the current cycle of gtod, which might be the TSC page
timestamp. Please rename cycle_now to tsc_timestamp in the call tree,
thanks.
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