Message-ID: <20250723175341.1284463-1-xin@zytor.com>
Date: Wed, 23 Jul 2025 10:53:18 -0700
From: "Xin Li (Intel)" <xin@...or.com>
To: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org
Cc: pbonzini@...hat.com, seanjc@...gle.com, corbet@....net, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, xin@...or.com, luto@...nel.org,
        peterz@...radead.org, andrew.cooper3@...rix.com, chao.gao@...el.com,
        hch@...radead.org
Subject: [PATCH v5 00/23] Enable FRED with KVM VMX

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 v4 of this patch set:
https://lore.kernel.org/lkml/20250328171205.2029296-1-xin@zytor.com/


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].

With the native CET patches now merged into the tip tree, the KVM CET
patch set is approaching maturity and is expected to be merged soon.

This v5 patch set is based on the kvm-x86-next-2025.07.21 tag of the
kvm-x86 repo.  Note that a few changes related to IA32_PL0_SSP are still
pending and are not included in this version.


Changes in v5:

1) Addressed review comments/feedbacks from Chao Gao, Christoph Hellwig,
   Dave Hansen, and Sean.

2) Added back a patch that checks before accessing VMCS fields via
   SHADOW_FIELD_R[OW] macros to avoid VMREAD/VMWRITE warnings, as some
   fields may not be supported on all CPUs.

3) Added "Tested-by: Xuelian Guo <xuelian.guo@...el.com>" to all patches.


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


Sean Christopherson (1):
  KVM: VMX: Add host MSR read/write helpers to streamline preemption
    logic

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: Allow VMX FRED controls
  KVM: nVMX: Add prerequisites to SHADOW_FIELD_R[OW] macros

Xin Li (Intel) (4):
  x86/cea: Export an API to get per CPU exception stacks for KVM to use
  KVM: VMX: Fix an indentation
  Documentation: kvm: Fix a section number typo
  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                 | 271 +++++++++++++++++++---
 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                    | 263 +++++++++++++++++++--
 arch/x86/kvm/vmx/vmx.h                    |  13 +-
 arch/x86/kvm/x86.c                        |  88 ++++++-
 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, 872 insertions(+), 97 deletions(-)


base-commit: 33f843444e28920d6e624c6c24637b4bb5d3c8de
-- 
2.50.1

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