Re: [PATCH RFC v1 17/20] KVM: x86: Prepare APX state setting in XCR0

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/10/25 19:01, Chang S. Bae wrote:
> Prepare the APX state enabling in XCR0 by implementing the previous
> placeholders and ensuring its readiness.
> 
> APX introduces EGPRs, tracked as XSTATE component 19. Like other
> XSAVE-managed states, EGPR availability is controlled through XCR0, and
> the registers are accessible only in 64-bit mode.
> 
> At this point, only VMX supports EGPRs. SVM will require corresponding
> extensions to handle EGPR indices.
> 
> The addition to the supported XCR0 mask should accompany guest CPUID
> exposure, which will be done separately.

You should also adjust set_xcr to reject setting BNDREGS and APX 
together (QEMU should also reject MPX and APX together in CPUID, but KVM 
doesn't care enough about invalid CPUID configuration).

Paolo

> 
> Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
> ---
> RFC note
> Not all callers may need to validate the XCR0 bit -- maybe a capability
> bit. However, every exit associated with EGPRs should already have that
> control bit set in the first place. Checking it explicitly does not
> charge additional cost, so I have this for consistency.
> ---
>   arch/x86/kvm/emulate.c        |  9 +++++++--
>   arch/x86/kvm/kvm_cache_regs.h |  1 +
>   arch/x86/kvm/kvm_emulate.h    |  1 +
>   arch/x86/kvm/svm/svm.c        |  7 ++++++-
>   arch/x86/kvm/vmx/vmx.h        |  9 ++++++++-
>   arch/x86/kvm/x86.c            | 11 +++++++++++
>   6 files changed, 34 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index f9381a4055d6..ba3020e6f469 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -4787,9 +4787,14 @@ static int decode_operand(struct x86_emulate_ctxt *ctxt, struct operand *op,
>   	return rc;
>   }
>   
> -static inline bool emul_egpr_enabled(struct x86_emulate_ctxt *ctxt __maybe_unused)
> +/* EGPR availability is controlled by the APX feature bit in XCR0. */
> +static inline bool emul_egpr_enabled(struct x86_emulate_ctxt *ctxt)
>   {
> -	return false;
> +	u64 xcr0;
> +
> +	ctxt->ops->get_xcr(ctxt, XCR_XFEATURE_ENABLED_MASK, &xcr0);
> +
> +	return xcr0 & XFEATURE_MASK_APX;
>   }
>   
>   int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int emulation_type)
> diff --git a/arch/x86/kvm/kvm_cache_regs.h b/arch/x86/kvm/kvm_cache_regs.h
> index 8ddb01191d6f..acdb3751317c 100644
> --- a/arch/x86/kvm/kvm_cache_regs.h
> +++ b/arch/x86/kvm/kvm_cache_regs.h
> @@ -3,6 +3,7 @@
>   #define ASM_KVM_CACHE_REGS_H
>   
>   #include <linux/kvm_host.h>
> +#include <asm/fpu/xcr.h>
>   
>   #define KVM_POSSIBLE_CR0_GUEST_BITS	(X86_CR0_TS | X86_CR0_WP)
>   #define KVM_POSSIBLE_CR4_GUEST_BITS				  \
> diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
> index cc16211d61f6..673a82532c78 100644
> --- a/arch/x86/kvm/kvm_emulate.h
> +++ b/arch/x86/kvm/kvm_emulate.h
> @@ -237,6 +237,7 @@ struct x86_emulate_ops {
>   	bool (*is_smm)(struct x86_emulate_ctxt *ctxt);
>   	int (*leave_smm)(struct x86_emulate_ctxt *ctxt);
>   	void (*triple_fault)(struct x86_emulate_ctxt *ctxt);
> +	int (*get_xcr)(struct x86_emulate_ctxt *ctxt, u32 index, u64 *xcr);
>   	int (*set_xcr)(struct x86_emulate_ctxt *ctxt, u32 index, u64 xcr);
>   
>   	gva_t (*get_untagged_addr)(struct x86_emulate_ctxt *ctxt, gva_t addr,
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 3aa2c37754ef..e6a082686000 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -5288,8 +5288,13 @@ static __init int svm_hardware_setup(void)
>   	}
>   	kvm_enable_efer_bits(EFER_NX);
>   
> +	/*
> +	 * APX introduces EGPRs, which require additional VMCB support.
> +	 * Disable APX until the necessary extensions are handled.
> +	 */
>   	kvm_caps.supported_xcr0 &= ~(XFEATURE_MASK_BNDREGS |
> -				     XFEATURE_MASK_BNDCSR);
> +				     XFEATURE_MASK_BNDCSR  |
> +				     XFEATURE_MASK_APX);
>   
>   	if (boot_cpu_has(X86_FEATURE_FXSR_OPT))
>   		kvm_enable_efer_bits(EFER_FFXSR);
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index 6cf1eb739caf..784aa0504dce 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -372,7 +372,14 @@ struct vmx_insn_info {
>   	union insn_info info;
>   };
>   
> -static inline bool vmx_egpr_enabled(struct kvm_vcpu *vcpu __maybe_unused) { return false; }
> +/*
> + * EGPR availability is controlled by the APX xfeature bit in XCR0 and is
> + * only accessible in 64-bit mode.
> + */
> +static inline bool vmx_egpr_enabled(struct kvm_vcpu *vcpu)
> +{
> +	return vcpu->arch.xcr0 & XFEATURE_MASK_APX && is_64_bit_mode(vcpu);
> +}
>   
>   static inline struct vmx_insn_info vmx_get_insn_info(struct kvm_vcpu *vcpu)
>   {
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4c8c2fc3bda6..e087db0f4153 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8843,6 +8843,16 @@ static void emulator_triple_fault(struct x86_emulate_ctxt *ctxt)
>   	kvm_make_request(KVM_REQ_TRIPLE_FAULT, emul_to_vcpu(ctxt));
>   }
>   
> +static int emulator_get_xcr(struct x86_emulate_ctxt *ctxt, u32 index, u64 *xcr)
> +{
> +	/* Only support XCR_XFEATURE_ENABLED_MASK now  */
> +	if (index != XCR_XFEATURE_ENABLED_MASK)
> +		return 1;
> +
> +	*xcr = emul_to_vcpu(ctxt)->arch.xcr0;
> +	return 0;
> +}
> +
>   static int emulator_set_xcr(struct x86_emulate_ctxt *ctxt, u32 index, u64 xcr)
>   {
>   	return __kvm_set_xcr(emul_to_vcpu(ctxt), index, xcr);
> @@ -8915,6 +8925,7 @@ static const struct x86_emulate_ops emulate_ops = {
>   	.is_smm              = emulator_is_smm,
>   	.leave_smm           = emulator_leave_smm,
>   	.triple_fault        = emulator_triple_fault,
> +	.get_xcr             = emulator_get_xcr,
>   	.set_xcr             = emulator_set_xcr,
>   	.get_untagged_addr   = emulator_get_untagged_addr,
>   	.is_canonical_addr   = emulator_is_canonical_addr,