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

[Isaku Yamahata]

On Tue, Nov 07, 2023 at 12:03:35PM -0800,
Jim Mattson <jmattson@xxxxxxxxxx> wrote:

> 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?

If you mean kvm kselftest, no.
I have
- TDX patched qemu
- kvm-unit-tests: test_apic_timer_one_shot() @ kvm-unit-tests/x86/apic.c
  TDX version is found at https://github.com/intel/kvm-unit-tests-tdx
  We're planning to upstream the changes for TDX

How far do we want to go?
- Run kvm-unit-tests with TDX. What I have right now.
- kvm-unit-tests: extend qemu for default VM case and update
  test_apic_timer_one_host()
- kselftest
  Right now kvm kselftest doesn't have test cases even for in-kernel IRQCHIP
  creation.
  - Add test cases to create/destory in-kernel IRQCHIP
  - Add KVM_ENABLE_CAPABILITY(KVM_CAP_X86_BUS_FREQUENCY_CONTROL) test cases
  - Test if apic timer frequency is as spcified.

-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxxxxxxxx>