This patch set enables the Intel flexible return and event delivery (FRED) architecture with KVM VMX to allow guests to utilize FRED. The FRED architecture defines simple new transitions that change privilege level (ring transitions). The FRED architecture was designed with the following goals: 1) Improve overall performance and response time by replacing event delivery through the interrupt descriptor table (IDT event delivery) and event return by the IRET instruction with lower latency transitions. 2) Improve software robustness by ensuring that event delivery establishes the full supervisor context and that event return establishes the full user context. The new transitions defined by the FRED architecture are FRED event delivery and, for returning from events, two FRED return instructions. FRED event delivery can effect a transition from ring 3 to ring 0, but it is used also to deliver events incident to ring 0. One FRED instruction (ERETU) effects a return from ring 0 to ring 3, while the other (ERETS) returns while remaining in ring 0. Collectively, FRED event delivery and the FRED return instructions are FRED transitions. Intel VMX architecture is extended to run FRED guests, and the major changes are: 1) New VMCS fields for FRED context management, which includes two new event data VMCS fields, eight new guest FRED context VMCS fields and eight new host FRED context VMCS fields. 2) VMX nested-exception support for proper virtualization of stack levels introduced with FRED architecture. Search for the latest FRED spec in most search engines with this search pattern: site:intel.com FRED (flexible return and event delivery) specification Following is the link to the v5 of this patch set: https://lore.kernel.org/lkml/20250723175341.1284463-1-xin@xxxxxxxxx/ Although FRED and CET supervisor shadow stacks are independent CPU features, FRED unconditionally includes FRED shadow stack pointer MSRs IA32_FRED_SSP[0123], and IA32_FRED_SSP0 is just an alias of the CET MSR IA32_PL0_SSP. IOW, the state management of MSR IA32_PL0_SSP becomes an overlap area, and Sean requested that FRED virtualization to land after CET virtualization [1]. This v6 patch set is based on the kvm-x86-next-2025.08.20 tag of the kvm-x86 repo + v13 of the KVM CET patch set, and also available at https://github.com/xinli-intel/linux-fred-public.git fred-kvm-v6 Changes in v6: 1) Return KVM_MSR_RET_UNSUPPORTED instead of 1 when FRED is not available (Chao Gao) 2) Handle MSR_IA32_PL0_SSP when FRED is enumerated but CET not. 3) Handle FRED MSR pre-vmenter save/restore (Chao Gao). 4) Save FRED MSRs of vmcs02 at VM-Exit even an L1 VMM clears SECONDARY_VM_EXIT_SAVE_IA32_FRED. 5) Save FRED MSRs in sync_vmcs02_to_vmcs12() instead of its rare version. [1]: https://lore.kernel.org/kvm/ZvQaNRhrsSJTYji3@xxxxxxxxxx/ Xin Li (18): KVM: VMX: Add support for the secondary VM exit controls KVM: VMX: Initialize VM entry/exit FRED controls in vmcs_config KVM: VMX: Disable FRED if FRED consistency checks fail KVM: VMX: Initialize VMCS FRED fields KVM: VMX: Set FRED MSR intercepts KVM: VMX: Save/restore guest FRED RSP0 KVM: VMX: Add support for FRED context save/restore KVM: x86: Add a helper to detect if FRED is enabled for a vCPU KVM: VMX: Virtualize FRED event_data KVM: VMX: Virtualize FRED nested exception tracking KVM: x86: Mark CR4.FRED as not reserved KVM: VMX: Dump FRED context in dump_vmcs() KVM: x86: Advertise support for FRED KVM: nVMX: Add support for the secondary VM exit controls KVM: nVMX: Add FRED VMCS fields to nested VMX context handling KVM: nVMX: Add FRED-related VMCS field checks KVM: nVMX: Add prerequisites to SHADOW_FIELD_R[OW] macros KVM: nVMX: Allow VMX FRED controls Xin Li (Intel) (2): x86/cea: Export an API to get per CPU exception stacks for KVM to use KVM: x86: Save/restore the nested flag of an exception Documentation/virt/kvm/api.rst | 21 +- Documentation/virt/kvm/x86/nested-vmx.rst | 19 ++ arch/x86/coco/sev/sev-nmi.c | 4 +- arch/x86/coco/sev/vc-handle.c | 2 +- arch/x86/include/asm/cpu_entry_area.h | 17 +- arch/x86/include/asm/kvm_host.h | 8 +- arch/x86/include/asm/msr-index.h | 1 + arch/x86/include/asm/vmx.h | 48 ++- arch/x86/include/uapi/asm/kvm.h | 4 +- arch/x86/kernel/cpu/common.c | 10 +- arch/x86/kernel/fred.c | 6 +- arch/x86/kernel/traps.c | 2 +- arch/x86/kvm/cpuid.c | 1 + arch/x86/kvm/kvm_cache_regs.h | 15 + arch/x86/kvm/svm/svm.c | 2 +- arch/x86/kvm/vmx/capabilities.h | 25 +- arch/x86/kvm/vmx/nested.c | 338 +++++++++++++++++++--- arch/x86/kvm/vmx/nested.h | 22 ++ arch/x86/kvm/vmx/vmcs.h | 1 + arch/x86/kvm/vmx/vmcs12.c | 19 ++ arch/x86/kvm/vmx/vmcs12.h | 38 +++ arch/x86/kvm/vmx/vmcs_shadow_fields.h | 37 ++- arch/x86/kvm/vmx/vmx.c | 240 ++++++++++++++- arch/x86/kvm/vmx/vmx.h | 54 +++- arch/x86/kvm/x86.c | 115 +++++++- arch/x86/kvm/x86.h | 8 +- arch/x86/mm/cpu_entry_area.c | 21 ++ arch/x86/mm/fault.c | 2 +- include/uapi/linux/kvm.h | 1 + 29 files changed, 976 insertions(+), 105 deletions(-) -- 2.50.1
