| Message ID | 6146ef9f9e5a17a1940b0efb571c5143b0e9ef8f.1711035400.git.reinette.chatre@intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: x86: Make bus clock frequency for vAPIC timer configurable | expand |
On Thu, 2024-03-21 at 09:37 -0700, Reinette Chatre wrote: > From: Isaku Yamahata <isaku.yamahata@intel.com> > > Add KVM_CAP_X86_APIC_BUS_FREQUENCY capability to configure the APIC > bus clock frequency for APIC timer emulation. > Allow KVM_ENABLE_CAPABILITY(KVM_CAP_X86_APIC_BUS_FREQUENCY) to set the > frequency in nanoseconds. When using this capability, the user space > VMM should configure CPUID leaf 0x15 to advertise the frequency. > > Vishal reported that the TDX guest kernel expects a 25MHz APIC bus > frequency but ends up getting interrupts at a significantly higher rate. > > The TDX architecture hard-codes the core crystal clock frequency to > 25MHz and mandates exposing it via CPUID leaf 0x15. The TDX architecture > does not allow the VMM to override the value. > > In addition, per Intel SDM: > "The APIC timer frequency will be the processor’s bus clock or core > crystal clock frequency (when TSC/core crystal clock ratio is > enumerated in CPUID leaf 0x15) divided by the value specified in > the divide configuration register." > > The resulting 25MHz APIC bus frequency conflicts with the KVM hardcoded > APIC bus frequency of 1GHz. > > The KVM doesn't enumerate CPUID leaf 0x15 to the guest unless the user > space VMM sets it using 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. As reported by Vishal the TDX guest > kernel expects a 25MHz timer frequency but gets timer interrupt more > frequently due to the 1GHz frequency used by KVM. > > 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 series). > Pro: It resembles the existing hardware. The recent Intel CPUs > adapts 25MHz. > Con: Require the VMM to emulate the APIC timer at 25MHz. > 2. Make the TDX architecture enumerate CPUID leaf 0x15 to configurable > frequency or not enumerate it. > Pro: Any APIC bus frequency is allowed. > Con: Deviates from TDX architecture. > 3. Make the TDX guest kernel use 1GHz when it's running on KVM. > Con: The kernel ignores CPUID leaf 0x15. > 4. Change CPUID leaf 0x15 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. > Con: It doesn't work with other VMMs as TDX isn't specific to KVM. > Con: Core crystal clock frequency is also used to calculate TSC > frequency. > Con: If it is configured to value different from hardware, it will > break the correctness of INTEL-PT Mini Time Count (MTC) packets > in TDs. > > Reported-by: Vishal Annapurve <vannapurve@google.com> > Closes: > https://lore.kernel.org/lkml/20231006011255.4163884-1-vannapurve@google.com/ Is Closes appropriate, given the issue Vishal hit was on non-upstream code? > Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com> > Co-developed-by: Reinette Chatre <reinette.chatre@intel.com> > Signed-off-by: Reinette Chatre <reinette.chatre@intel.com> Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Hi Rick, On 4/16/2024 10:08 AM, Edgecombe, Rick P wrote: > On Thu, 2024-03-21 at 09:37 -0700, Reinette Chatre wrote: >> From: Isaku Yamahata <isaku.yamahata@intel.com> >> >> Add KVM_CAP_X86_APIC_BUS_FREQUENCY capability to configure the APIC >> bus clock frequency for APIC timer emulation. >> Allow KVM_ENABLE_CAPABILITY(KVM_CAP_X86_APIC_BUS_FREQUENCY) to set the >> frequency in nanoseconds. When using this capability, the user space >> VMM should configure CPUID leaf 0x15 to advertise the frequency. >> >> Vishal reported that the TDX guest kernel expects a 25MHz APIC bus >> frequency but ends up getting interrupts at a significantly higher rate. >> >> The TDX architecture hard-codes the core crystal clock frequency to >> 25MHz and mandates exposing it via CPUID leaf 0x15. The TDX architecture >> does not allow the VMM to override the value. >> >> In addition, per Intel SDM: >> "The APIC timer frequency will be the processor’s bus clock or core >> crystal clock frequency (when TSC/core crystal clock ratio is >> enumerated in CPUID leaf 0x15) divided by the value specified in >> the divide configuration register." >> >> The resulting 25MHz APIC bus frequency conflicts with the KVM hardcoded >> APIC bus frequency of 1GHz. >> >> The KVM doesn't enumerate CPUID leaf 0x15 to the guest unless the user >> space VMM sets it using 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. As reported by Vishal the TDX guest >> kernel expects a 25MHz timer frequency but gets timer interrupt more >> frequently due to the 1GHz frequency used by KVM. >> >> 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 series). >> Pro: It resembles the existing hardware. The recent Intel CPUs >> adapts 25MHz. >> Con: Require the VMM to emulate the APIC timer at 25MHz. >> 2. Make the TDX architecture enumerate CPUID leaf 0x15 to configurable >> frequency or not enumerate it. >> Pro: Any APIC bus frequency is allowed. >> Con: Deviates from TDX architecture. >> 3. Make the TDX guest kernel use 1GHz when it's running on KVM. >> Con: The kernel ignores CPUID leaf 0x15. >> 4. Change CPUID leaf 0x15 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. >> Con: It doesn't work with other VMMs as TDX isn't specific to KVM. >> Con: Core crystal clock frequency is also used to calculate TSC >> frequency. >> Con: If it is configured to value different from hardware, it will >> break the correctness of INTEL-PT Mini Time Count (MTC) packets >> in TDs. >> >> Reported-by: Vishal Annapurve <vannapurve@google.com> >> Closes: >> https://lore.kernel.org/lkml/20231006011255.4163884-1-vannapurve@google.com/ > > Is Closes appropriate, given the issue Vishal hit was on non-upstream code? The issue that Vishal encountered was root-caused to the APIC bus frequency conflict between KVM and TDX that has not changed since the original report. It would be great if this submission can get confirmation that this series addresses the issues encountered originally. From what I can tell there has not been any formal accept/reject of this solution (apart from agreeing that it is the right approach [1]) since the original report. Reinette [1] https://lore.kernel.org/lkml/CAGtprH8-jiC+wsy2LgmZimrRUT6kuntD6EJso2Mvx5Y42za9Dw@mail.gmail.com/
On 3/22/2024 12:37 AM, Reinette Chatre wrote: ... > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > index 0b5a33ee71ee..20080fe4b8ee 100644 > --- a/Documentation/virt/kvm/api.rst > +++ b/Documentation/virt/kvm/api.rst > @@ -8063,6 +8063,23 @@ error/annotated fault. > > See KVM_EXIT_MEMORY_FAULT for more information. > > +7.35 KVM_CAP_X86_APIC_BUS_FREQUENCY As sean mentioned it previous comment, I would be the one prefers KVM_CAP_X86_APIC_BUS_CYCLES_NS Depending on whether people get hung up on nanoseconds not being a "frequency", either KVM_CAP_X86_APIC_BUS_FREQUENCY or KVM_CAP_X86_APIC_BUS_CYCLES_NS.
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index 0b5a33ee71ee..20080fe4b8ee 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -8063,6 +8063,23 @@ error/annotated fault. See KVM_EXIT_MEMORY_FAULT for more information. +7.35 KVM_CAP_X86_APIC_BUS_FREQUENCY +----------------------------------- + +:Architectures: x86 +:Target: VM +:Parameters: args[0] is the desired APIC bus clock rate, in nanoseconds +:Returns: 0 on success, -EINVAL if args[0] contains an invalid value for the + frequency or if any vCPUs have been created, -ENXIO if a virtual + local APIC has not been created using KVM_CREATE_IRQCHIP. + +This capability sets VM's APIC bus clock frequency, used by KVM's in-kernel +virtual APIC when emulating APIC timers. KVM's default value can be retrieved +by KVM_CHECK_EXTENSION. + +Note: Userspace is responsible for correctly configuring CPUID 0x15, a.k.a. the +core crystal clock frequency, if a non-zero CPUID 0x15 is exposed to the guest. + 8. Other capabilities. ====================== diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 23f25799c760..bf52c0ca7a56 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -4713,6 +4713,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) case KVM_CAP_MEMORY_FAULT_INFO: r = 1; break; + case KVM_CAP_X86_APIC_BUS_FREQUENCY: + r = APIC_BUS_CYCLE_NS_DEFAULT; + break; case KVM_CAP_EXIT_HYPERCALL: r = KVM_EXIT_HYPERCALL_VALID_MASK; break; @@ -6726,6 +6729,30 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm, } mutex_unlock(&kvm->lock); break; + case KVM_CAP_X86_APIC_BUS_FREQUENCY: { + u64 bus_cycle_ns = cap->args[0]; + u64 unused; + + r = -EINVAL; + /* + * Guard against overflow in tmict_to_ns(). 128 is the highest + * divide value that can be programmed in APIC_TDCR. + */ + if (!bus_cycle_ns || + check_mul_overflow((u64)U32_MAX * 128, bus_cycle_ns, &unused)) + break; + + r = 0; + mutex_lock(&kvm->lock); + if (!irqchip_in_kernel(kvm)) + r = -ENXIO; + else if (kvm->created_vcpus) + r = -EINVAL; + else + kvm->arch.apic_bus_cycle_ns = bus_cycle_ns; + mutex_unlock(&kvm->lock); + break; + } default: r = -EINVAL; break; diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h index 2190adbe3002..49da7ec97bd5 100644 --- a/include/uapi/linux/kvm.h +++ b/include/uapi/linux/kvm.h @@ -917,6 +917,7 @@ struct kvm_enable_cap { #define KVM_CAP_MEMORY_ATTRIBUTES 233 #define KVM_CAP_GUEST_MEMFD 234 #define KVM_CAP_VM_TYPES 235 +#define KVM_CAP_X86_APIC_BUS_FREQUENCY 236 struct kvm_irq_routing_irqchip { __u32 irqchip;