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

[Jim Mattson]

On Mon, Dec 18, 2023 at 5:40 PM Isaku Yamahata
<isaku.yamahata@xxxxxxxxxxxxxxx> wrote:
>
> On Thu, Dec 14, 2023 at 08:41:43AM -0800,
> Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> > On Thu, Dec 14, 2023, Maxim Levitsky wrote:
> > > On Wed, 2023-12-13 at 15:10 -0800, Sean Christopherson wrote:
> > > > 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.
> > >
> > > I assume that TDX doesn't allow to change the CPUID 0x15 leaf.
> >
> > Correct.  I meant to call that out below, but left my sentence half-finished.  It
> > was supposed to say:
> >
> >   I halfheartedly floated the idea of "fixing" the TDX module/architecture to either
> >   use 1Ghz as the base frequency or to allow configuring the base frequency
> >   advertised to the guest.
> >
> > > > 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.
> > > >
> > > > https://lore.kernel.org/all/ZSnIKQ4bUavAtBz6@xxxxxxxxxx
> > > >
> > >
> > > Best regards,
> > >     Maxim Levitsky
>
> The followings are the updated version of the commit message.
>
>
> KVM: x86: Make the hardcoded APIC bus frequency VM variable
>
> The TDX architecture hard-codes the APIC bus frequency to 25MHz in the
> CPUID leaf 0x15.  The
> TDX mandates it to be exposed and doesn't allow the VMM to override
> its value.  The KVM APIC timer emulation hard-codes the frequency to
> 1GHz.  It doesn't unconditionally enumerate it to the guest unless the
> user space VMM sets the CPUID leaf 0x15 by KVM_SET_CPUID.
>
> If the CPUID leaf 0x15 is enumerated, the guest kernel uses it as the
> APIC bus frequency.  If not, the guest kernel measures the frequency
> based on other known timers like the ACPI timer or the legacy PIT.
> The TDX guest kernel gets timer interrupt more times by 1GHz / 25MHz.
>
> To ensure that the guest doesn't have a conflicting view of the APIC
> bus frequency, allow the userspace to tell KVM to use the same
> frequency that TDX mandates instead of the default 1Ghz.
>
> There are several options to address this.
> 1. Make the KVM able to configure APIC bus frequency (This patch).
>    Pros: It resembles the existing hardware.  The recent Intel CPUs
>    adapts 25MHz.
>    Cons: Require the VMM to emulate the APIC timer at 25MHz.
> 2. Make the TDX architecture enumerate CPUID 0x15 to configurable
>    frequency or not enumerate it.
>    Pros: Any APIC bus frequency is allowed.
>    Cons: Deviation from the real hardware.
> 3. Make the TDX guest kernel use 1GHz when it's running on KVM.
>    Cons: The kernel ignores CPUID leaf 0x15.

4. Change CPUID.15H under TDX to report the crystal clock frequency as 1 GHz.
Pro: This has been the virtual APIC frequency for KVM guests for 13 years.
Pro: This requires changing only one hard-coded constant in TDX.

I see no compelling reason to complicate KVM with support for
configurable APIC frequencies, and I see no advantages to doing so.