KVM: X86: Make bus clock frequency for vapic timer (bus lock -> bus clock) (was Re: [PATCH 0/2] KVM: X86: Make bus lock frequency for vapic timer) configurable

[Isaku Yamahata]

I meant bus clock frequency, not bus lock. Sorry for typo.

On Tue, Nov 07, 2023 at 11:22:32AM -0800,
isaku.yamahata@xxxxxxxxx wrote:

> From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> 
> 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_FREUQNCY_CONTROL) to set the
> frequency.  When using this capability, the user space VMM should configure
> CPUID[0x15] to advertise the frequency.
> 
> TDX virtualizes CPUID[0x15] for the core crystal clock to be 25MHz.  The
> x86 KVM hardcodes its freuqncy 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.
> 
> There are options to reconcile the mismatch.  1) Make apic bus clock frequency
> configurable (this patch).  2) TDX KVM code adjusts TMICT value.  This is hacky
> and it results in losing MSB bits from 32 bit width to 30 bit width.  3). Make
> the guest kernel use tsc deadline timer instead of acpi oneshot/periodic timer.
> This is guest kernel choice.  It's out of control of VMM.
> 
> [1] https://lore.kernel.org/lkml/20231006011255.4163884-1-vannapurve@xxxxxxxxxx/
> 
> Isaku Yamahata (2):
>   KVM: x86: Make the hardcoded APIC bus frequency vm variable
>   KVM: X86: Add a capability to configure bus frequency for APIC timer
> 
>  arch/x86/include/asm/kvm_host.h |  2 ++
>  arch/x86/kvm/hyperv.c           |  2 +-
>  arch/x86/kvm/lapic.c            |  6 ++++--
>  arch/x86/kvm/lapic.h            |  4 ++--
>  arch/x86/kvm/x86.c              | 16 ++++++++++++++++
>  include/uapi/linux/kvm.h        |  1 +
>  6 files changed, 26 insertions(+), 5 deletions(-)
> 
> -- 
> 2.25.1
> 

-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxxxxxxxx>