linux-kernel - Re: [PATCH V5 2/4] KVM: x86: Make nsec per APIC bus cycle a VM variable

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Date: Mon, 29 Apr 2024 12:23:33 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Reinette Chatre <reinette.chatre@...el.com>, isaku.yamahata@...el.com,
 pbonzini@...hat.com, erdemaktas@...gle.com, vkuznets@...hat.com,
 seanjc@...gle.com, vannapurve@...gle.com, jmattson@...gle.com,
 mlevitsk@...hat.com, chao.gao@...el.com, rick.p.edgecombe@...el.com
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V5 2/4] KVM: x86: Make nsec per APIC bus cycle a VM
 variable

On 4/26/2024 6:07 AM, Reinette Chatre wrote:
> From: Isaku Yamahata <isaku.yamahata@...el.com>
> 
> Introduce the VM variable "nanoseconds per APIC bus cycle" in
> preparation to make the APIC bus frequency configurable.
> 
> The TDX architecture hard-codes the core crystal clock frequency to
> 25MHz and mandates exposing it via CPUID leaf 0x15. The TDX architecture
> does not allow the VMM to override the value.
> 
> In addition, per Intel SDM:
>      "The APIC timer frequency will be the processor’s bus clock or core
>       crystal clock frequency (when TSC/core crystal clock ratio is
>       enumerated in CPUID leaf 0x15) divided by the value specified in
>       the divide configuration register."
> 
> The resulting 25MHz APIC bus frequency conflicts with the KVM hardcoded
> APIC bus frequency of 1GHz.
> 
> Introduce the VM variable "nanoseconds per APIC bus cycle" to prepare
> for allowing userspace to tell KVM to use the frequency that TDX mandates
> instead of the default 1Ghz. Doing so ensures that the guest doesn't have
> a conflicting view of the APIC bus frequency.

Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>

> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> Reviewed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> [reinette: rework changelog]
> Signed-off-by: Reinette Chatre <reinette.chatre@...el.com>
> ---
> Changes v5:
> - Add Rick's Reviewed-by tag.
> 
> Changes v4:
> - Reword changelog to address comments related to "bus clock" vs
>    "core crystal clock" frequency. (Xiaoyao)
> - Typo in changelog ("APIC APIC" -> "APIC").
> - Change logic "APIC bus cycles per nsec" -> "nanoseconds per
>    APIC bus cycle".
> 
> Changes V3:
> - Update commit message.
> - Dropped apic_bus_frequency according to Maxim Levitsky.
> 
> Changes v2:
> - No change.
> 
>   arch/x86/include/asm/kvm_host.h | 1 +
>   arch/x86/kvm/hyperv.c           | 3 ++-
>   arch/x86/kvm/lapic.c            | 6 ++++--
>   arch/x86/kvm/lapic.h            | 2 +-
>   arch/x86/kvm/x86.c              | 1 +
>   5 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 1d13e3cd1dc5..f2735582c7e0 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1358,6 +1358,7 @@ struct kvm_arch {
>   
>   	u32 default_tsc_khz;
>   	bool user_set_tsc;
> +	u64 apic_bus_cycle_ns;
>   
>   	seqcount_raw_spinlock_t pvclock_sc;
>   	bool use_master_clock;
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index 1030701db967..5c31e715d2ad 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -1737,7 +1737,8 @@ static int kvm_hv_get_msr(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata,
>   		data = (u64)vcpu->arch.virtual_tsc_khz * 1000;
>   		break;
>   	case HV_X64_MSR_APIC_FREQUENCY:
> -		data = div64_u64(1000000000ULL, APIC_BUS_CYCLE_NS);
> +		data = div64_u64(1000000000ULL,
> +				 vcpu->kvm->arch.apic_bus_cycle_ns);
>   		break;
>   	default:
>   		kvm_pr_unimpl_rdmsr(vcpu, msr);
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index cf37586f0466..3e66a0a95999 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1547,7 +1547,8 @@ static u32 apic_get_tmcct(struct kvm_lapic *apic)
>   		remaining = 0;
>   
>   	ns = mod_64(ktime_to_ns(remaining), apic->lapic_timer.period);
> -	return div64_u64(ns, (APIC_BUS_CYCLE_NS * apic->divide_count));
> +	return div64_u64(ns, (apic->vcpu->kvm->arch.apic_bus_cycle_ns *
> +			      apic->divide_count));
>   }
>   
>   static void __report_tpr_access(struct kvm_lapic *apic, bool write)
> @@ -1965,7 +1966,8 @@ static void start_sw_tscdeadline(struct kvm_lapic *apic)
>   
>   static inline u64 tmict_to_ns(struct kvm_lapic *apic, u32 tmict)
>   {
> -	return (u64)tmict * APIC_BUS_CYCLE_NS * (u64)apic->divide_count;
> +	return (u64)tmict * apic->vcpu->kvm->arch.apic_bus_cycle_ns *
> +		(u64)apic->divide_count;
>   }
>   
>   static void update_target_expiration(struct kvm_lapic *apic, uint32_t old_divisor)
> diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> index a20cb006b6c8..51e09f5a7fc5 100644
> --- a/arch/x86/kvm/lapic.h
> +++ b/arch/x86/kvm/lapic.h
> @@ -16,7 +16,7 @@
>   #define APIC_DEST_NOSHORT		0x0
>   #define APIC_DEST_MASK			0x800
>   
> -#define APIC_BUS_CYCLE_NS       1
> +#define APIC_BUS_CYCLE_NS_DEFAULT	1
>   
>   #define APIC_BROADCAST			0xFF
>   #define X2APIC_BROADCAST		0xFFFFFFFFul
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e9ef1fa4b90b..10e6315103f4 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12629,6 +12629,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>   	raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
>   
>   	kvm->arch.default_tsc_khz = max_tsc_khz ? : tsc_khz;
> +	kvm->arch.apic_bus_cycle_ns = APIC_BUS_CYCLE_NS_DEFAULT;
>   	kvm->arch.guest_can_read_msr_platform_info = true;
>   	kvm->arch.enable_pmu = enable_pmu;
>