Re: [PATCH v2 1/3] KVM: x86: Make the hardcoded APIC bus frequency vm variable

[Jim Mattson]

On Wed, Dec 13, 2023 at 3:10 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Thu, Dec 14, 2023, Maxim Levitsky wrote:
> > On Mon, 2023-11-13 at 20:35 -0800, isaku.yamahata@xxxxxxxxx wrote:
> > > From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> > >
> > > TDX virtualizes the advertised APIC bus frequency to be 25MHz.
> >
> > Can you explain a bit better why TDX needs this? I am not familiar
> > with TDX well enough yet to fully understand.
>
> TDX (the module/architecture) hardcodes the core crystal frequency to 25Mhz,
> whereas KVM hardcodes the APIC bus frequency to 1Ghz.  And TDX (again, the module)
> *unconditionally* enumerates CPUID 0x15 to TDX guests, i.e. _tells_ the guest that
> the frequency is 25MHz regardless of what the VMM/hypervisor actually emulates.
> And so the guest skips calibrating the APIC timer, which results in the guest
> scheduling timer interrupts waaaaaaay too frequently, i.e. the guest ends up
> gettings interrupts at 40x the rate it wants.
>
> Upstream KVM's non-TDX behavior is fine, because KVM doesn't advertise support
> for CPUID 0x15, i.e. doesn't announce to host userspace that it's safe to expose
> CPUID 0x15 to the guest.  Because TDX makes exposing CPUID 0x15 mandatory, KVM
> needs to be taught to correctly emulate the guest's APIC bus frequency, a.k.a.
> the TDX guest core crystal frequency of 25Mhz.

Aside from placating a broken guest infrastructure that ignores a
17-year old contract between KVM and its guests, what are the
advantages to supporting a range of APIC bus frequencies?

> I halfheartedly floated the idea of "fixing" the TDX module/architecture to either
> use 1Ghz as the base frequency (off list), but it definitely isn't a hill worth
> dying on since the KVM changes are relatively simple.

Not making the KVM changes is even simpler. :)