Re: [PATCH RFC v1 07/20] KVM: nVMX: Support the extended instruction info field

Paolo Bonzini
2025-11-11T17:49:03Z

On 11/10/25 19:01, Chang S. Bae wrote:
> Define the VMCS field offset for the extended instruction information and
> handle it for nested VMX.
>
> When EGPRs are available, VMX provides a new 64-bit field to extend the
> legacy instruction information, allowing access to the higher register
> indices. Then, nested VMX needs to propagate this field between L1 and
> L2.
>
> The EGPR checker will be implemented later.
>
> Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
> ---
> RFC note:
> During the draft, I brought up the offset definition initially for
> non-nested VMX primarily. Then, I realized the switching helper affects
> nVMX code anyway. Due to this dependency, this change is placed first
> together with the offset definition.
> ---
> arch/x86/include/asm/vmx.h | 2 ++
> arch/x86/kvm/vmx/nested.c | 2 ++
> arch/x86/kvm/vmx/vmcs12.c | 1 +
> arch/x86/kvm/vmx/vmcs12.h | 3 ++-
> arch/x86/kvm/vmx/vmx.h | 2 ++
> 5 files changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
> index c85c50019523..ab0684948c56 100644
> --- a/arch/x86/include/asm/vmx.h
> +++ b/arch/x86/include/asm/vmx.h
> @@ -264,6 +264,8 @@ enum vmcs_field {
> PID_POINTER_TABLE_HIGH = 0x00002043,
> GUEST_PHYSICAL_ADDRESS = 0x00002400,
> GUEST_PHYSICAL_ADDRESS_HIGH = 0x00002401,
> + EXTENDED_INSTRUCTION_INFO = 0x00002406,
> + EXTENDED_INSTRUCTION_INFO_HIGH = 0x00002407,
> VMCS_LINK_POINTER = 0x00002800,
> VMCS_LINK_POINTER_HIGH = 0x00002801,
> GUEST_IA32_DEBUGCTL = 0x00002802,
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 97ec8e594155..3442610a6b70 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -4798,6 +4798,8 @@ static void prepare_vmcs12(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
> vmcs12->vm_exit_intr_info = exit_intr_info;
> vmcs12->vm_exit_instruction_len = exit_insn_len;
> vmcs12->vmx_instruction_info = vmcs_read32(VMX_INSTRUCTION_INFO);
> + if (vmx_egpr_enabled(vcpu))
> + vmcs12->extended_instruction_info = vmcs_read64(EXTENDED_INSTRUCTION_INFO);

From patch 17:

+static inline bool vmx_egpr_enabled(struct kvm_vcpu *vcpu)
+{
+ return vcpu->arch.xcr0 & XFEATURE_MASK_APX && is_64_bit_mode(vcpu);
+}

but here you must not check XCR0, the extended instruction information
field is always available. The spec says "A non-Intel® APX enabled VMM
is free to continue using the legacy definition of the field, since lack
of Intel® APX enabling will guarantee that regIDs are only 4-bits,
maximum" but you can also use the extended instruction information field
if you want. So, I'd make this also static_cpu_has(X86_FEATURE_APX).

Paolo

>
> /*
> * According to spec, there's no need to store the guest's
> diff --git a/arch/x86/kvm/vmx/vmcs12.c b/arch/x86/kvm/vmx/vmcs12.c
> index 4233b5ca9461..ea2b690a419e 100644
> --- a/arch/x86/kvm/vmx/vmcs12.c
> +++ b/arch/x86/kvm/vmx/vmcs12.c
> @@ -53,6 +53,7 @@ const unsigned short vmcs12_field_offsets[] = {
> FIELD64(XSS_EXIT_BITMAP, xss_exit_bitmap),
> FIELD64(ENCLS_EXITING_BITMAP, encls_exiting_bitmap),
> FIELD64(GUEST_PHYSICAL_ADDRESS, guest_physical_address),
> + FIELD64(EXTENDED_INSTRUCTION_INFO, extended_instruction_info),
> FIELD64(VMCS_LINK_POINTER, vmcs_link_pointer),
> FIELD64(GUEST_IA32_DEBUGCTL, guest_ia32_debugctl),
> FIELD64(GUEST_IA32_PAT, guest_ia32_pat),
> diff --git a/arch/x86/kvm/vmx/vmcs12.h b/arch/x86/kvm/vmx/vmcs12.h
> index 4ad6b16525b9..2146e45aaade 100644
> --- a/arch/x86/kvm/vmx/vmcs12.h
> +++ b/arch/x86/kvm/vmx/vmcs12.h
> @@ -71,7 +71,7 @@ struct __packed vmcs12 {
> u64 pml_address;
> u64 encls_exiting_bitmap;
> u64 tsc_multiplier;
> - u64 padding64[1]; /* room for future expansion */
> + u64 extended_instruction_info;
> /*
> * To allow migration of L1 (complete with its L2 guests) between
> * machines of different natural widths (32 or 64 bit), we cannot have
> @@ -261,6 +261,7 @@ static inline void vmx_check_vmcs12_offsets(void)
> CHECK_OFFSET(pml_address, 312);
> CHECK_OFFSET(encls_exiting_bitmap, 320);
> CHECK_OFFSET(tsc_multiplier, 328);
> + CHECK_OFFSET(extended_instruction_info, 336);
> CHECK_OFFSET(cr0_guest_host_mask, 344);
> CHECK_OFFSET(cr4_guest_host_mask, 352);
> CHECK_OFFSET(cr0_read_shadow, 360);
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index 64a0772c883c..b8da6ebc35dc 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -372,6 +372,8 @@ struct vmx_insn_info {
> union insn_info info;
> };
>
> +static inline bool vmx_egpr_enabled(struct kvm_vcpu *vcpu __maybe_unused) { return false; }
> +
> static inline struct vmx_insn_info vmx_get_insn_info(struct kvm_vcpu *vcpu __maybe_unused)
> {
> struct vmx_insn_info insn;