linux-kernel - Re: [PATCH v5 4/7] KVM: SVM: Software reserved fields

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Message-ID: <67feoyvmmf2sl34kikk3btrfcedafax2pazht5tplxyeb5rtv7@eakih2vxt2xc>
Date: Thu, 11 Sep 2025 22:35:32 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Vineeth Pillai <viremana@...ux.microsoft.com>
Cc: Lan Tianyu <Tianyu.Lan@...rosoft.com>, 
	Michael Kelley <mikelley@...rosoft.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Sean Christopherson <seanjc@...gle.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, 
	Tom Lendacky <thomas.lendacky@....com>, Wanpeng Li <wanpengli@...cent.com>, 
	Jim Mattson <jmattson@...gle.com>, Joerg Roedel <joro@...tes.org>, Wei Liu <wei.liu@...nel.org>, 
	Stephen Hemminger <sthemmin@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>, 
	"H. Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	"K. Y. Srinivasan" <kys@...rosoft.com>, x86@...nel.org, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org, 
	Venkatesh Srinivas <venkateshs@...gle.com>, .@google.com
Subject: Re: [PATCH v5 4/7] KVM: SVM: Software reserved fields

On Thu, Jun 03, 2021 at 03:14:37PM +0000, Vineeth Pillai wrote:
> SVM added support for certain reserved fields to be used by
> software or hypervisor. Add the following reserved fields:
>   - VMCB offset 0x3e0 - 0x3ff
>   - Clean bit 31
>   - SVM intercept exit code 0xf0000000
> 
> Later patches will make use of this for supporting Hyper-V
> nested virtualization enhancements.
> 
> Signed-off-by: Vineeth Pillai <viremana@...ux.microsoft.com>
> ---
>  arch/x86/include/asm/svm.h      |  9 +++++++--
>  arch/x86/include/uapi/asm/svm.h |  3 +++
>  arch/x86/kvm/svm/svm.h          | 17 +++++++++++++++--
>  3 files changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
> index 772e60efe243..e322676039f4 100644
> --- a/arch/x86/include/asm/svm.h
> +++ b/arch/x86/include/asm/svm.h
> @@ -156,6 +156,12 @@ struct __attribute__ ((__packed__)) vmcb_control_area {
>  	u64 avic_physical_id;	/* Offset 0xf8 */
>  	u8 reserved_7[8];
>  	u64 vmsa_pa;		/* Used for an SEV-ES guest */
> +	u8 reserved_8[720];
> +	/*
> +	 * Offset 0x3e0, 32 bytes reserved
> +	 * for use by hypervisor/software.
> +	 */
> +	u8 reserved_sw[32];
>  };
>  
>  
> @@ -314,7 +320,7 @@ struct ghcb {
>  
>  
>  #define EXPECTED_VMCB_SAVE_AREA_SIZE		1032
> -#define EXPECTED_VMCB_CONTROL_AREA_SIZE		272
> +#define EXPECTED_VMCB_CONTROL_AREA_SIZE		1024
>  #define EXPECTED_GHCB_SIZE			PAGE_SIZE
>  
>  static inline void __unused_size_checks(void)
> @@ -326,7 +332,6 @@ static inline void __unused_size_checks(void)
>  
>  struct vmcb {
>  	struct vmcb_control_area control;
> -	u8 reserved_control[1024 - sizeof(struct vmcb_control_area)];
>  	struct vmcb_save_area save;
>  } __packed;
>  
> diff --git a/arch/x86/include/uapi/asm/svm.h b/arch/x86/include/uapi/asm/svm.h
> index 554f75fe013c..efa969325ede 100644
> --- a/arch/x86/include/uapi/asm/svm.h
> +++ b/arch/x86/include/uapi/asm/svm.h
> @@ -110,6 +110,9 @@
>  #define SVM_VMGEXIT_GET_AP_JUMP_TABLE		1
>  #define SVM_VMGEXIT_UNSUPPORTED_EVENT		0x8000ffff
>  
> +/* Exit code reserved for hypervisor/software use */
> +#define SVM_EXIT_SW				0xf0000000

Apologies for reviving this 2021 thread, but it seems like the APM says
in Table C-1. SVM Intercept Codes that the host reserved value is
F000_000h.

APM typo or wrong KVM definition?