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

[Jim Mattson]

On Wed, Dec 20, 2023 at 9:44 PM Xiaoyao Li <xiaoyao.li@xxxxxxxxx> wrote:
>
> On 12/21/2023 6:07 AM, Sean Christopherson wrote:
> > On Tue, Dec 19, 2023, Isaku Yamahata wrote:
> >> On Mon, Dec 18, 2023 at 07:53:45PM -0800, Jim Mattson <jmattson@xxxxxxxxxx> wrote:
> >>>> 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.
> >
> > I don't buy this as a valid Con.  TDX is one gigantic deviation from real hardware,
> > and since TDX obviously can't guarantee the APIC timer is emulated at the correct
> > frequency, there can't possibly be any security benefits.  If this were truly a
> > Con that anyone cared about, we would have gotten patches to "fix" KVM a long time
> > ago.
> >
> > If the TDX module wasn't effectively hardware-defined software, i.e. was actually
> > able to adapt at the speed of software, then fixing this in TDX would be a complete
> > no-brainer.
> >
> > The KVM uAPI required to play nice is relatively minor, so I'm not totally opposed
> > to adding it.  But I totally agree with Jim that forcing KVM to change 13+ years
> > of behavior just because someone at Intel decided that 25MHz was a good number is
> > ridiculous.
>
> I believe 25MHz was chosen because it's the value from hardware that
> supports TDX and it is not going to change for the following known
> generations that support TDX.
>
> It's mainly the core crystal frequency. Yes, it also represents the APIC
> frequency when it's enumerated in CPUID 0x15. However, it also relates
> other things, like intel-pt MTC Freq. If it is configured to other value
> different from hardware, I think it will break the correctness of
> INTEL-PT MTC packets in TDs.

LOL! That suggests that no one is really using KVM's Intel PT virtualization.

This is certainly a compelling reason for having a variable frequency
virtual APIC. Thank you!

> >>>> 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.
> >>
> >> Because TDX isn't specific to KVM, it should work with other VMM technologies.
> >> If we'd like to go for this route, the frequency would be configurable.  What
> >> frequency should be acceptable securely is obscure.  25MHz has long history with
> >> the real hardware.
> >
>