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 <isaku.yamahata@linux.intel.com>]

On Thu, Nov 09, 2023 at 07:55:45AM -0800,
Sean Christopherson <seanjc@google.com> wrote:

> On Wed, Nov 08, 2023, Isaku Yamahata wrote:
> > On Tue, Nov 07, 2023 at 12:03:35PM -0800, Jim Mattson <jmattson@google.com> wrote:
> > > 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()
> 
> Hrm, I'm not sure that we can do a whole lot for test_apic_timer_one_shot().  Or
> rather, I'm not sure it's worth the effort to try and add coverage beyond what's
> already there.
> 
> As for TDX, *if* we extend KUT, please don't make it depend on TDX.  Very few people
> have access to TDX platforms and anything CoCo is pretty much guaranteed to be harder
> to debug.

It made the test cases work with TDX + UEFI bios by adjusting command line to
invoke qemu.  And skip unsuitable tests.
Maybe we can generalize the way to twist qemu command line.


> > - kselftest
> >   Right now kvm kselftest doesn't have test cases even for in-kernel IRQCHIP
> >   creation.
> 
> Selftests always create an in-kernel APIC.  And I think selftests are perfectly
> suited to complement the coverage provided by KUT.  Specifically, the failure
> scenario for this is that KVM emulates at 1Ghz whereas TDX advertises 25Mhz, i.e.
> the test case we want is to verify that the APIC timer doesn't expire early.
> 
> There's no need for any APIC infrastructure, e.g. a selftest doesn't even need to
> handle an interrupt.  Get the TSC frequency from KVM, program up an arbitrary APIC
> bus clock frequency, set TMICT such that it expires waaaay in the future, and then
> verify that the APIC timer counts reasonably close to the programmed frequency.
> E.g. if the test sets the bus clock to 25Mhz, the "drift" due to KVM counting at
> 1Ghz should be super obvious.

Oh, only check the register value without interrupt. Good idea.  Let me give it
a try.
-- 
Isaku Yamahata <isaku.yamahata@linux.intel.com>