Re: [PATCH v7 02/10] x86/fred: Pass event data to the NMI entry point from KVM

[Xin Li <xin@zytor.com>]

On 6/12/2025 2:48 PM, Sohil Mehta wrote:
> Extend the FRED NMI entry point from KVM to take an extra argument to
> allow KVM to invoke the FRED event dispatch framework with event data.
> 
> This API is used to pass the NMI-source bitmap for NMI-induced VM exits.
> Read the VMCS exit qualification field to get the NMI-source information
> and store it as event data precisely in the format expected by the FRED
> event framework.
> 
> Read the VMCS exit qualification unconditionally since almost all
> upcoming CPUs are expected to enable FRED and NMI-source together. In
> the rare case that NMI-source isn't enabled, the extra VMREAD would be
> harmless since the exit qualification is expected to be zero.
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Originally-by: Zeng Guang <guang.zeng@intel.com>
> Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>

A couple of nits below, otherwise:

Reviewed-by: Xin Li (Intel) <xin@zytor.com>

> ---
> v7: Pass the event data from KVM only for NMI. (Sean)
> 
> v6: No change
> 
> v5: Read the VMCS exit qualification unconditionally. (Sean)
>      Combine related patches into one.
> ---
>   arch/x86/entry/entry_64_fred.S |  2 +-
>   arch/x86/include/asm/fred.h    | 11 ++++++-----
>   arch/x86/kvm/vmx/vmx.c         |  2 +-
>   3 files changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S
> index 29c5c32c16c3..1c9c6e036233 100644
> --- a/arch/x86/entry/entry_64_fred.S
> +++ b/arch/x86/entry/entry_64_fred.S
> @@ -93,7 +93,7 @@ SYM_FUNC_START(asm_fred_entry_from_kvm)
>   	 * +--------+-----------------+
>   	 */
>   	push $0				/* Reserved, must be 0 */
> -	push $0				/* Event data, 0 for IRQ/NMI */
> +	push %rsi			/* Event data for NMI */

Maybe a bit more accurate?

/* Event data, NMI-source bitmap only so far */

>   	push %rdi			/* fred_ss handed in by the caller */
>   	push %rbp
>   	pushf
> diff --git a/arch/x86/include/asm/fred.h b/arch/x86/include/asm/fred.h
> index 552332ca060c..bccf4a3c06b8 100644
> --- a/arch/x86/include/asm/fred.h
> +++ b/arch/x86/include/asm/fred.h
> @@ -66,14 +66,14 @@ static __always_inline unsigned long fred_event_data(struct pt_regs *regs)
>   
>   void asm_fred_entrypoint_user(void);
>   void asm_fred_entrypoint_kernel(void);
> -void asm_fred_entry_from_kvm(struct fred_ss);
> +void asm_fred_entry_from_kvm(struct fred_ss ss, unsigned long edata);
>   
>   __visible void fred_entry_from_user(struct pt_regs *regs);
>   __visible void fred_entry_from_kernel(struct pt_regs *regs);
>   __visible void __fred_entry_from_kvm(struct pt_regs *regs);
>   
>   /* Must be called from noinstr code, thus __always_inline */
> -static __always_inline void fred_nmi_from_kvm(void)
> +static __always_inline void fred_nmi_from_kvm(unsigned long edata)
>   {
>   	struct fred_ss ss = {
>   		.ss	= __KERNEL_DS,
> @@ -83,7 +83,7 @@ static __always_inline void fred_nmi_from_kvm(void)
>   		.lm	= 1,
>   	};
>   
> -	asm_fred_entry_from_kvm(ss);
> +	asm_fred_entry_from_kvm(ss, edata);
>   }
>   
>   static inline void fred_irq_from_kvm(unsigned int vector)
> @@ -95,7 +95,8 @@ static inline void fred_irq_from_kvm(unsigned int vector)
>   		.lm	= 1,
>   	};
>   
> -	asm_fred_entry_from_kvm(ss);
> +	/* Event data is always zero for IRQ */

/* Event data not used for IRQ thus 0 */