Re: [RFC PATCH v3 02/21] KVM: x86: Improve accuracy of KVM clock when TSC scaling is in force

[Sean Christopherson <seanjc@google.com>]

On Wed, May 22, 2024, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> The kvm_guest_time_update() function scales the host TSC frequency to
> the guest's using kvm_scale_tsc() and the v->arch.l1_tsc_scaling_ratio
> scaling ratio previously calculated for that vCPU. Then calcuates the
> scaling factors for the KVM clock itself based on that guest TSC
> frequency.
> 
> However, it uses kHz as the unit when scaling, and then multiplies by
> 1000 only at the end.
> 
> With a host TSC frequency of 3000MHz and a guest set to 2500MHz, the
> result of kvm_scale_tsc() will actually come out at 2,499,999kHz. So
> the KVM clock advertised to the guest is based on a frequency of
> 2,499,999,000 Hz.
> 
> By using Hz as the unit from the beginning, the KVM clock would be based
> on a more accurate frequency of 2,499,999,999 Hz in this example.
> 
> Fixes: 78db6a503796 ("KVM: x86: rewrite handling of scaled TSC for kvmclock")
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> Reviewed-by: Paul Durrant <paul@xen.org>
> ---
>  arch/x86/include/asm/kvm_host.h |  2 +-
>  arch/x86/kvm/x86.c              | 17 +++++++++--------
>  arch/x86/kvm/xen.c              |  2 +-
>  3 files changed, 11 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 01c69840647e..8440c4081727 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -887,7 +887,7 @@ struct kvm_vcpu_arch {
>  
>  	gpa_t time;
>  	struct pvclock_vcpu_time_info hv_clock;
> -	unsigned int hw_tsc_khz;
> +	unsigned int hw_tsc_hz;

Isn't there an overflow issue here?  The local variable is a 64-bit value, but
kvm_vcpu_arch.hw_tsc_hz is a 32-bit value.  And unless I'm having an even worse
review week than I thought, a guest TSC frequency > 4Ghz will get truncated.

>  	struct gfn_to_pfn_cache pv_time;
>  	/* set guest stopped flag in pvclock flags field */
>  	bool pvclock_set_guest_stopped_request;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2d2619d3eee4..23281c508c27 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3215,7 +3215,8 @@ static void kvm_setup_guest_pvclock(struct kvm_vcpu *v,
>  
>  static int kvm_guest_time_update(struct kvm_vcpu *v)
>  {
> -	unsigned long flags, tgt_tsc_khz;
> +	unsigned long flags;
> +	uint64_t tgt_tsc_hz;

s/uint64_t/u64 for kernel code.  There are more than a few uses of uint64_t in
KVM, but u64 is far and away the dominant flavor.

> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index 5a83a8154b79..014048c22652 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -2273,7 +2273,7 @@ void kvm_xen_update_tsc_info(struct kvm_vcpu *vcpu)
>  
>  	entry = kvm_find_cpuid_entry_index(vcpu, function, 2);
>  	if (entry)
> -		entry->eax = vcpu->arch.hw_tsc_khz;
> +		entry->eax = vcpu->arch.hw_tsc_hz / 1000;

And if hw_tsc_hz is a u64, this will need to use div_u64() to play nice with
32-bit kernels.