linux-kernel - RE: [PATCH v5 34/34] KVM: x86/vmx: execute "int $2" to handle NMI in NMI caused VM exits when FRED is enabled

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Date:   Tue, 7 Mar 2023 22:00:40 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     "Li, Xin3" <xin3.li@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: RE: [PATCH v5 34/34] KVM: x86/vmx: execute "int $2" to handle NMI in
 NMI caused VM exits when FRED is enabled

> Execute "int $2" to handle NMI in NMI caused VM exits when FRED is enabled.
> 
> Like IRET for IDT, ERETS/ERETU are required to end the NMI handler for FRED
> to unblock NMI ASAP (w/ bit 28 of CS set). And there are 2 approaches to
> invoke the FRED NMI handler:
> 1) execute "int $2", let the h/w do the job.
> 2) create a FRED NMI stack frame on the current kernel stack with ASM,
>    and then jump to fred_entrypoint_kernel in arch/x86/entry/entry_64_fred.S.
> 
> 1) is preferred as we want less ASM.
> 
> Tested-by: Shan Kang <shan.kang@...el.com>
> Signed-off-by: Xin Li <xin3.li@...el.com>
> ---
> 
> Changes since v4:
> *) Do NOT use the term "injection", which in the KVM context means to
>    reinject an event into the guest (Sean Christopherson).
> *) Add the explanation of why to execute "int $2" to invoke the NMI handler
>    in NMI caused VM exits (Sean Christopherson).

Sean,

Do you have any further issue with the last 2 VMX patches?

If not, would you ack them?

Thanks!
  Xin


> ---
>  arch/x86/kvm/vmx/vmx.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 3ebeaab34b2e..4f12ead2266b 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7229,7 +7229,16 @@ static noinstr void vmx_vcpu_enter_exit(struct
> kvm_vcpu *vcpu,
>  	if ((u16)vmx->exit_reason.basic == EXIT_REASON_EXCEPTION_NMI &&
>  	    is_nmi(vmx_get_intr_info(vcpu))) {
>  		kvm_before_interrupt(vcpu, KVM_HANDLING_NMI);
> -		vmx_do_nmi_irqoff();
> +		/*
> +		 * Like IRET for IDT, ERETS/ERETU are required to end the NMI
> +		 * handler for FRED to unblock NMI ASAP (w/ bit 28 of CS set).
> +		 *
> +		 * Invoke the FRED NMI handler through executing "int $2".
> +		 */
> +		if (cpu_feature_enabled(X86_FEATURE_FRED))
> +			asm volatile("int $2");
> +		else
> +			vmx_do_nmi_irqoff();
>  		kvm_after_interrupt(vcpu);
>  	}
> 
> --
> 2.34.1