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.
