linux-kernel - Re: [PATCH v3 2/4] KVM: x86: Make the APIC bus cycles per nsec VM variable

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Date: Thu, 21 Dec 2023 19:39:13 +0200
From: Maxim Levitsky <mlevitsk@...hat.com>
To: Isaku Yamahata <isaku.yamahata@...el.com>, kvm@...r.kernel.org, 
 linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>, 
 erdemaktas@...gle.com, Sean Christopherson <seanjc@...gle.com>, Vishal
 Annapurve <vannapurve@...gle.com>, Jim Mattson <jmattson@...gle.com>,
 Xiaoyao Li <xiaoyao.li@...el.com>
Cc: isaku.yamahata@...il.com
Subject: Re: [PATCH v3 2/4] KVM: x86: Make the APIC bus cycles per nsec VM
 variable

On Tue, 2023-12-19 at 00:34 -0800, Isaku Yamahata wrote:
> Introduce the VM variable of the APIC cycles per nano second as the
> preparation to make the APIC APIC bus frequency configurable.
> 
> The TDX architecture hard-codes the APIC bus frequency to 25MHz in the
> CPUID leaf 0x15.  The TDX mandates it to be exposed and doesn't allow the
> VMM to override its value.  The KVM APIC timer emulation hard-codes the
> frequency to 1GHz.  To ensure that the guest doesn't have a conflicting
> view of the APIC bus frequency, allow the userspace to tell KVM to use the
> same frequency that TDX mandates instead of the default 1Ghz.
> 
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> ---
> Changes v3:
> - Update commit message.
> - Dropped apic_bus_frequency according to Maxim Levitsky.

Looks good to me, thanks!



> 
> 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 d7036982332e..45ee7a1d13f6 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1334,6 +1334,7 @@ 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 a40ca2fef58c..37ff31c18ad1 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -1687,7 +1687,8 @@ 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 245b20973cae..73956b0ac1f1 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1542,7 +1542,8 @@ 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)
> @@ -1960,7 +1961,8 @@ 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 1a3aaa7dafae..d7d865f7c847 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12466,6 +12466,7 @@ 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;
>  


Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky