Re: [PATCH v2 2/3] KVM: X86: Add a capability to configure bus frequency for APIC timer

[Maxim Levitsky <mlevitsk@redhat.com>]

On Mon, 2023-11-13 at 20:35 -0800, isaku.yamahata@intel.com wrote:
> From: Isaku Yamahata <isaku.yamahata@intel.com>
> 
> Add KVM_CAP_X86_BUS_FREQUENCY_CONTROL capability to configure the core
> crystal clock (or processor's bus clock) for APIC timer emulation.  Allow
> KVM_ENABLE_CAPABILITY(KVM_CAP_X86_BUS_FREQUENCY_CONTROL) to set the
> frequency.
> 
> TDX virtualizes CPUID[0x15] for the core crystal clock to be 25MHz.  The
> x86 KVM hardcodes its frequency for APIC timer to be 1GHz.  This mismatch
> causes the vAPIC timer to fire earlier than the guest expects. [1] The KVM
> APIC timer emulation uses hrtimer, whose unit is nanosecond.  Make the
> parameter configurable for conversion from the TMICT value to nanosecond.
> 
> This patch doesn't affect the TSC deadline timer emulation.  The TSC
> deadline emulation path records its expiring TSC value and calculates the
> expiring time in nanoseconds.  The APIC timer emulation path calculates the
> TSC value from the TMICT register value and uses the TSC deadline timer
> path.  This patch touches the APIC timer-specific code but doesn't touch
> common logic.

Nitpick: To be honest IMHO the paragraph about tsc deadline is redundant, because by definition (x86 spec)
the tsc deadline timer doesn't use APIC bus frequency.

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

Best regards,
	Maxim Levitsky

> 
> [1] https://lore.kernel.org/lkml/20231006011255.4163884-1-vannapurve@google.com/
> Reported-by: Vishal Annapurve <vannapurve@google.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> ---
> Changes v2:
> - Add check if vcpu isn't created.
> - Add check if lapic chip is in-kernel emulation.
> - Fix build error for i386
> - Add document to api.rst
> - typo in the commit message
> ---
>  Documentation/virt/kvm/api.rst | 14 ++++++++++++++
>  arch/x86/kvm/x86.c             | 35 ++++++++++++++++++++++++++++++++++
>  include/uapi/linux/kvm.h       |  1 +
>  3 files changed, 50 insertions(+)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 7025b3751027..cc976df2651e 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -7858,6 +7858,20 @@ This capability is aimed to mitigate the threat that malicious VMs can
>  cause CPU stuck (due to event windows don't open up) and make the CPU
>  unavailable to host or other VMs.
>  
> +7.34 KVM_CAP_X86_BUS_FREQUENCY_CONTROL
> +--------------------------------------
> +
> +:Architectures: x86
> +:Target: VM
> +:Parameters: args[0] is the value of apic bus clock frequency
> +:Returns: 0 on success, -EINVAL if args[0] contains invalid value for the
> +          frequency, or -ENXIO if virtual local APIC isn't enabled by
> +          KVM_CREATE_IRQCHIP, or -EBUSY if any vcpu is created.
> +
> +This capability sets the APIC bus clock frequency (or core crystal clock
> +frequency) for kvm to emulate APIC in the kernel.  The default value is 1000000
> +(1GHz).
> +
>  8. Other capabilities.
>  ======================
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a9f4991b3e2e..a8fb862c4f8e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4625,6 +4625,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  	case KVM_CAP_ENABLE_CAP:
>  	case KVM_CAP_VM_DISABLE_NX_HUGE_PAGES:
>  	case KVM_CAP_IRQFD_RESAMPLE:
> +	case KVM_CAP_X86_BUS_FREQUENCY_CONTROL:
>  		r = 1;
>  		break;
>  	case KVM_CAP_EXIT_HYPERCALL:
> @@ -6616,6 +6617,40 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>  		}
>  		mutex_unlock(&kvm->lock);
>  		break;
> +	case KVM_CAP_X86_BUS_FREQUENCY_CONTROL: {
> +		u64 bus_frequency = cap->args[0];
> +		u64 bus_cycle_ns;
> +
> +		if (!bus_frequency)
> +			return -EINVAL;
> +		/* CPUID[0x15] only support 32bits.  */
> +		if (bus_frequency != (u32)bus_frequency)
> +			return -EINVAL;
> +
> +		/* Cast to avoid 64bit division on 32bit platform. */
> +		bus_cycle_ns = 1000000000UL / (u32)bus_frequency;
> +		if (!bus_cycle_ns)
> +			return -EINVAL;
> +
> +		r = 0;
> +		mutex_lock(&kvm->lock);
> +		/*
> +		 * Don't allow to change the frequency dynamically during vcpu
> +		 * running to avoid potentially bizarre behavior.
> +		 */
> +		if (kvm->created_vcpus)
> +			r = -EBUSY;
> +		/* This is for in-kernel vAPIC emulation. */
> +		else if (!irqchip_in_kernel(kvm))
> +			r = -ENXIO;
> +
> +		if (!r) {
> +			kvm->arch.apic_bus_cycle_ns = bus_cycle_ns;
> +			kvm->arch.apic_bus_frequency = bus_frequency;
> +		}
> +		mutex_unlock(&kvm->lock);
> +		return r;
> +	}
>  	default:
>  		r = -EINVAL;
>  		break;
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 211b86de35ac..d74a057df173 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1201,6 +1201,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE 228
>  #define KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES 229
>  #define KVM_CAP_ARM_SUPPORTED_REG_MASK_RANGES 230
> +#define KVM_CAP_X86_BUS_FREQUENCY_CONTROL 231
>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>