On Wed, May 22, 2024, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> Let the callers pass the host TSC value in as an explicit parameter.
>
> This leaves some fairly obviously stupid code, which using this function
> to compare the guest TSC at some *other* time, with the newly-minted TSC
> value from rdtsc(). Unless it's being used to measure *elapsed* time,
> that isn't very sensible.
>
> In this case, "obviously stupid" is an improvement over being non-obviously
> so.
>
> No functional change intended.
>
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> ---
> arch/x86/kvm/x86.c | 14 ++++++++------
> 1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ef3cd6113037..ea59694d712a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2601,11 +2601,12 @@ u64 kvm_scale_tsc(u64 tsc, u64 ratio)
> return _tsc;
> }
>
> -static u64 kvm_compute_l1_tsc_offset(struct kvm_vcpu *vcpu, u64 target_tsc)
> +static u64 kvm_compute_l1_tsc_offset(struct kvm_vcpu *vcpu, u64 host_tsc,
> + u64 target_tsc)
Would it make sense to have a __kvm_compute_l1_tsc_offset() version that takes
in the host TSC, and then this?
static u64 kvm_compute_l1_tsc_offset(struct kvm_vcpu *vcpu, u64 target_tsc)
{
return __kvm_compute_l1_tsc_offset(vcpu, rdtsc(), target_tsc);
}
Hmm, or maybe a better option would be:
static u64 kvm_compute_current_l1_tsc_offset(struct kvm_vcpu *vcpu,
u64 target_tsc)
{
return kvm_compute_l1_tsc_offset(vcpu, rdtsc(), target_tsc);
}
Meh, after typing those out, I don't like either one. Let's keep it how you
wrote it, I think there's quite a bit of added readability by forcing callers to
provide the host TSC.
> {
> u64 tsc;
>
> - tsc = kvm_scale_tsc(rdtsc(), vcpu->arch.l1_tsc_scaling_ratio);
> + tsc = kvm_scale_tsc(host_tsc, vcpu->arch.l1_tsc_scaling_ratio);
>
> return target_tsc - tsc;
Opportunistically drop "tsc" too? E.g.
return target_tsc -
kvm_scale_tsc(host_tsc, vcpu->arch.l1_tsc_scaling_ratio);
or
return target_tsc - kvm_scale_tsc(host_tsc, vcpu->arch.l1_tsc_scaling_ratio);
I find either of those much easier to read.
