Re: [RFC PATCH v3 10/21] KVM: x86: Fix software TSC upscaling in kvm_update_guest_time()

[Paul Durrant]

On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> There was some confusion in kvm_update_guest_time() when software needs
> to advance the guest TSC.
>
> In master clock mode, there are two points of time which need to be taken
> into account. First there is the master clock reference point, stored in
> kvm->arch.master_kernel_ns (and associated host TSC ->master_cycle_now).
> Secondly, there is the time *now*, at the point kvm_update_guest_time()
> is being called.
>
> With software TSC upscaling, the guest TSC is getting further and further
> ahead of the host TSC as time elapses. So at time "now", the guest TSC
> should be further ahead of the host, than it was at master_kernel_ns.
>
> The adjustment in kvm_update_guest_time() was not taking that into
> account, and was only advancing the guest TSC by the appropriate amount
> for master_kernel_ns, *not* the current time.
>
> Fix it to calculate them both correctly.
>
> Since the KVM clock reference point in master_kernel_ns might actually
> be *earlier* than the reference point used for the guest TSC
> (vcpu->last_tsc_nsec), this might lead to a negative delta. Fix the
> compute_guest_tsc() function to cope with negative numbers, which
> then means there is no need to force a master clock update when the
> guest TSC is written.
>
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx > ---
>   arch/x86/kvm/x86.c | 73 +++++++++++++++++++++++++++++++++++-----------
>   1 file changed, 56 insertions(+), 17 deletions(-)

Reviewed-by: Paul Durrant <paul@xxxxxxx>