On Wed, May 22, 2024, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxx>
> Reviewed-by: Paul Durrant <paul@xxxxxxx>
> ---
> 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.
