linux-kernel - Re: [PATCH V4 2/4] KVM: x86: Make nsec per APIC bus cycle a VM variable

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Date: Tue, 16 Apr 2024 17:08:04 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "jmattson@...gle.com" <jmattson@...gle.com>, "Gao, Chao"
	<chao.gao@...el.com>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"vkuznets@...hat.com" <vkuznets@...hat.com>, "Annapurve, Vishal"
	<vannapurve@...gle.com>, "Chatre, Reinette" <reinette.chatre@...el.com>, "Li,
 Xiaoyao" <xiaoyao.li@...el.com>, "Aktas, Erdem" <erdemaktas@...gle.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "mlevitsk@...hat.com"
	<mlevitsk@...hat.com>, "Yamahata, Isaku" <isaku.yamahata@...el.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V4 2/4] KVM: x86: Make nsec per APIC bus cycle a VM
 variable

On Thu, 2024-03-21 at 09:37 -0700, 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.
> 
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> [reinette: rework changelog]
> Signed-off-by: Reinette Chatre <reinette.chatre@...el.com>

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>