Re: [RFC PATCH v3 12/21] KVM: x86: Remove implicit rdtsc() from kvm_compute_l1_tsc_offset()

[Sean Christopherson <seanjc@google.com>]

On Wed, May 22, 2024, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> 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@amazon.co.uk>
> ---
>  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.