Re: 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

[Jim Mattson]

On Tue, Nov 7, 2023 at 11:29 AM Isaku Yamahata
<isaku.yamahata@xxxxxxxxxxxxxxx> wrote:
>
> 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

I think I know the answer, but do you have any tests for this new feature?