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

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

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

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