[PATCH v8 00/21] Enable FRED with KVM VMX

Xin Li (Intel)
2025-10-14T01:11:14Z

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

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].

Since CET virtualization has landed with the release of v6.18-rc1, I'm
submitting v8 of FRED virtualization.

Changes in v8:
* Rebased on v6.18-rc1.
* Relocate secondary_vm_exit_controls to the last u64 padding field in
vmcs12 (Isaku).
* Make the newly added FRED fields 64-bit aligned in vmcs12 (Isaku).
* Remove changes to Documentation/virt/kvm/x86/nested-vmx.rst.
* Update KVM_CAP_EXCEPTION_NESTED_FLAG, as the number used in v7 is
occupied by another new cap.

Following is the link to v7 of this patch set:
https://lore.kernel.org/lkml/20250829153149.2871901-1-xin@zytor.com/

[1]: https://lore.kernel.org/kvm/ZvQaNRhrsSJTYji3@google.com/

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 saving and restoring FRED MSRs
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) (3):
x86/cea: Prefix event stack names with ESTACK_
x86/cea: Export API for per-CPU exception stacks for KVM
KVM: x86: Save/restore the nested flag of an exception

Documentation/virt/kvm/api.rst | 21 +-
arch/x86/coco/sev/noinstr.c | 4 +-
arch/x86/coco/sev/vc-handle.c | 2 +-
arch/x86/include/asm/cpu_entry_area.h | 75 +++---
arch/x86/include/asm/kvm_host.h | 13 +-
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/dumpstack_64.c | 14 +-
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 | 40 ++-
arch/x86/kvm/vmx/vmcs_shadow_fields.h | 37 ++-
arch/x86/kvm/vmx/vmx.c | 247 +++++++++++++++++--
arch/x86/kvm/vmx/vmx.h | 54 +++-
arch/x86/kvm/x86.c | 131 +++++++++-
arch/x86/kvm/x86.h | 8 +-
arch/x86/mm/cpu_entry_area.c | 37 ++-
arch/x86/mm/fault.c | 2 +-
include/uapi/linux/kvm.h | 1 +
29 files changed, 1036 insertions(+), 144 deletions(-)

base-commit: 3a8660878839faadb4f1a6dd72c3179c1df56787
--
2.51.0