linux-kernel - Re: [PATCH v2 09/11] KVM: SVM: Add VNMI bit definition

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Message-ID: <Y9mZRU3nbz6ru2lS@google.com>
Date:   Tue, 31 Jan 2023 22:42:13 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, Sandipan Das <sandipan.das@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        Jiaxi Chen <jiaxi.chen@...ux.intel.com>,
        Babu Moger <babu.moger@....com>, linux-kernel@...r.kernel.org,
        Jing Liu <jing2.liu@...el.com>,
        Wyes Karny <wyes.karny@....com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Santosh Shukla <santosh.shukla@....com>
Subject: Re: [PATCH v2 09/11] KVM: SVM: Add VNMI bit definition

On Tue, Nov 29, 2022, Maxim Levitsky wrote:
> From: Santosh Shukla <santosh.shukla@....com>
> 
> VNMI exposes 3 capability bits (V_NMI, V_NMI_MASK, and V_NMI_ENABLE) to
> virtualize NMI and NMI_MASK, Those capability bits are part of
> VMCB::intr_ctrl -
> V_NMI(11) - Indicates whether a virtual NMI is pending in the guest.
> V_NMI_MASK(12) - Indicates whether virtual NMI is masked in the guest.
> V_NMI_ENABLE(26) - Enables the NMI virtualization feature for the guest.
> 
> When Hypervisor wants to inject NMI, it will set V_NMI bit, Processor
> will clear the V_NMI bit and Set the V_NMI_MASK which means the Guest is
> handling NMI, After the guest handled the NMI, The processor will clear
> the V_NMI_MASK on the successful completion of IRET instruction Or if
> VMEXIT occurs while delivering the virtual NMI.
> 
> To enable the VNMI capability, Hypervisor need to program
> V_NMI_ENABLE bit 1.
> 
> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> Signed-off-by: Santosh Shukla <santosh.shukla@....com>
> ---
>  arch/x86/include/asm/svm.h | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
> index cb1ee53ad3b189..26d6f549ce2b46 100644
> --- a/arch/x86/include/asm/svm.h
> +++ b/arch/x86/include/asm/svm.h
> @@ -203,6 +203,13 @@ struct __attribute__ ((__packed__)) vmcb_control_area {
>  #define X2APIC_MODE_SHIFT 30
>  #define X2APIC_MODE_MASK (1 << X2APIC_MODE_SHIFT)
>  
> +#define V_NMI_PENDING_SHIFT 11
> +#define V_NMI_PENDING (1 << V_NMI_PENDING_SHIFT)
> +#define V_NMI_MASK_SHIFT 12
> +#define V_NMI_MASK (1 << V_NMI_MASK_SHIFT)

Argh, more KVM warts.  The existing INT_CTL defines all use "mask" in the name,
so looking at V_NMI_MASK in the context of other code reads "vNMI is pending",
not "vNMIs are blocked".

IMO, the existing _MASK terminology is the one that's wrong, but there's an absurd
amount of prior art in svm.h :-(

And the really annoying one is V_INTR_MASKING_MASK, which IIRC says "virtual INTR
masking is enabled", not "virtual INTRs are blocked".

So maybe call this V_NMI_BLOCKING_MASK?  And tack on _MASK too the others (even
though I agree it's ugly).

> +#define V_NMI_ENABLE_SHIFT 26
> +#define V_NMI_ENABLE (1 << V_NMI_ENABLE_SHIFT)

Hrm.  I think I would prefer to keep the defines ordered by bit position.  Knowing
that there's an enable bit isn't all that critical for understanding vNMI pending
and blocked.