Message-ID: <20250802001520.3142577-1-xin@zytor.com>
Date: Fri,  1 Aug 2025 17:15:16 -0700
From: "Xin Li (Intel)" <xin@...or.com>
To: linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc: pbonzini@...hat.com, seanjc@...gle.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, xin@...or.com, chao.gao@...el.com
Subject: [PATCH v2 0/4] KVM: VMX: Handle the immediate form of MSR instructions

This patch set handles two newly introduced VM exit reasons associated
with the immediate form of MSR instructions to ensure proper
virtualization of these instructions.

The immediate form of MSR access instructions are primarily motivated
by performance, not code size: by having the MSR number in an immediate,
it is available *much* earlier in the pipeline, which allows the
hardware much more leeway about how a particular MSR is handled.

For proper virtualization of the immediate form of MSR instructions,
Intel VMX architecture adds the following changes:

  1) The immediate form of RDMSR uses VM exit reason 84.

  2) The immediate form of WRMSRNS uses VM exit reason 85.

  3) For both VM exit reasons 84 and 85, the exit qualification is set
     to the MSR address causing the VM exit.

  4) Bits 3 ~ 6 of the VM exit instruction information field represent
     the operand register used in the immediate form of MSR instruction.

  5) The VM-exit instruction length field records the size of the
     immediate form of the MSR instruction.

Note: The VMX specification for the immediate form of MSR instructions
was inadvertently omitted from the last published ISE, but it will be
included in the upcoming edition.

Linux bare metal support of the immediate form of MSR instructions is
still under development; however, the KVM support effort is proceeding
independently of the bare metal implementation.


Link to v1:
https://lore.kernel.org/lkml/20250730174605.1614792-1-xin@zytor.com/


Changes in v2:
*) Added nested MSR bitmap check for the two new MSR-related VM exit
   reasons (Chao).
*) Shortened function names that still convey enough information
   (Chao & Sean).
*) Removed VCPU_EXREG_EDX_EAX as it unnecessarily exposes details of a
   specific flow across KVM (Sean).
*) Implemented a separate userspace completion callback for the
   immediate form RDMSR (Sean).
*) Passed MSR data directly to __kvm_emulate_wrmsr() instead of the
   encoded general-purpose register containing it (Sean).
*) Merged modifications to x86.c and vmx.c within the same patch to
   facilitate easier code review (Sean).
*) Moved fastpath support in a separate patch, i.e., patch 3 (Sean).
*) Cleared the immediate form MSR capability in SVM in patch 4 (Sean).


Xin Li (Intel) (4):
  x86/cpufeatures: Add a CPU feature bit for MSR immediate form
    instructions
  KVM: VMX: Handle the immediate form of MSR instructions
  KVM: VMX: Support the immediate form WRMSRNS in fastpath
  KVM: x86: Advertise support for the immediate form of MSR instructions

 arch/x86/include/asm/cpufeatures.h |  1 +
 arch/x86/include/asm/kvm_host.h    |  4 ++
 arch/x86/include/uapi/asm/vmx.h    |  6 +-
 arch/x86/kernel/cpu/scattered.c    |  1 +
 arch/x86/kvm/cpuid.c               |  6 +-
 arch/x86/kvm/reverse_cpuid.h       |  5 ++
 arch/x86/kvm/svm/svm.c             |  8 ++-
 arch/x86/kvm/vmx/nested.c          | 13 ++++-
 arch/x86/kvm/vmx/vmx.c             | 26 ++++++++-
 arch/x86/kvm/vmx/vmx.h             |  5 ++
 arch/x86/kvm/x86.c                 | 92 ++++++++++++++++++++++--------
 arch/x86/kvm/x86.h                 |  3 +-
 12 files changed, 139 insertions(+), 31 deletions(-)


base-commit: 33f843444e28920d6e624c6c24637b4bb5d3c8de
-- 
2.50.1

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