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Message-ID: <20250821223630.984383-1-xin@zytor.com>
Date: Thu, 21 Aug 2025 15:36:09 -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 v6 00/20] 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 v5 of this patch set:
https://lore.kernel.org/lkml/20250723175341.1284463-1-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].
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@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 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
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