linux-kernel - Re: [PATCH v2 1/2] KVM: nVMX: Disallow access to vmcs12 fields that aren't supported by "hardware"

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Message-ID: <777040ff-8064-4b19-8bd0-f4262e6c2e68@intel.com>
Date: Wed, 31 Dec 2025 16:36:55 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, Xin Li
 <xin@...or.com>, Chao Gao <chao.gao@...el.com>,
 Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: Re: [PATCH v2 1/2] KVM: nVMX: Disallow access to vmcs12 fields that
 aren't supported by "hardware"

On 12/31/2025 6:02 AM, Sean Christopherson wrote:
> +/*
> + * Indexing into the vmcs12 uses the VMCS encoding rotated left by 6 as a very
> + * rudimentary compression of the range of indices.  The compression ratio is
> + * good enough to allow KVM to use a (very sparsely populated) array without
> + * wasting too much memory, while the "algorithm" is fast enough to be used to
> + * lookup vmcs12 fields on-demand, e.g. for emulation.
> + */
>   #define ROL16(val, n) ((u16)(((u16)(val) << (n)) | ((u16)(val) >> (16 - (n)))))
> +#define VMCS12_IDX_TO_ENC(idx) ((u16)(((u16)(idx) >> 6) | ((u16)(idx) << 10)))

Put them together is really good.

And ROL16(val, 16-n) is exactly undoing ROL16(val, n), so that we can 
further do

---8<---
diff --git a/arch/x86/kvm/vmx/vmcs.h b/arch/x86/kvm/vmx/vmcs.h
index 98281e019e38..be2f410f82cd 100644
--- a/arch/x86/kvm/vmx/vmcs.h
+++ b/arch/x86/kvm/vmx/vmcs.h
@@ -19,7 +19,8 @@
   * lookup vmcs12 fields on-demand, e.g. for emulation.
   */
  #define ROL16(val, n) ((u16)(((u16)(val) << (n)) | ((u16)(val) >> (16 
- (n)))))
-#define VMCS12_IDX_TO_ENC(idx) ((u16)(((u16)(idx) >> 6) | ((u16)(idx) 
<< 10)))
+#define ENC_TO_VMCS12_IDX(enc) ROL16(enc, 6)
+#define VMCS12_IDX_TO_ENC(idx) ROL16(idx,10)

  struct vmcs_hdr {
         u32 revision_id:31;
diff --git a/arch/x86/kvm/vmx/vmcs12.c b/arch/x86/kvm/vmx/vmcs12.c
index b92db4768346..1ebe67c384ad 100644
--- a/arch/x86/kvm/vmx/vmcs12.c
+++ b/arch/x86/kvm/vmx/vmcs12.c
@@ -4,10 +4,10 @@
  #include "vmcs12.h"

  #define VMCS12_OFFSET(x) offsetof(struct vmcs12, x)
-#define FIELD(number, name)    [ROL16(number, 6)] = VMCS12_OFFSET(name)
+#define FIELD(number, name)    [ENC_TO_VMCS12_IDX(number)] = 
VMCS12_OFFSET(name)
  #define FIELD64(number, name)                                          \
         FIELD(number, name),                                            \
-       [ROL16(number##_HIGH, 6)] = VMCS12_OFFSET(name) + sizeof(u32)
+       [ENC_TO_VMCS12_IDX(number##_HIGH)] = VMCS12_OFFSET(name) + 
sizeof(u32)

  static const u16 kvm_supported_vmcs12_field_offsets[] __initconst = {
         FIELD(VIRTUAL_PROCESSOR_ID, virtual_processor_id),