linux-kernel - Re: [PATCH 2/2] KVM: SVM: Enable Virtual GIF feature

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Message-ID: <20170816195312.GB32542@flask>
Date:   Wed, 16 Aug 2017 21:53:12 +0200
From:   Radim Krcmar <rkrcmar@...hat.com>
To:     Janakarajan Natarajan <Janakarajan.Natarajan@....com>
Cc:     kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Andy Lutomirski <luto@...nel.org>,
        Tony Luck <tony.luck@...el.com>, Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Yazen Ghannam <yazen.ghannam@....com>
Subject: Re: [PATCH 2/2] KVM: SVM: Enable Virtual GIF feature

2017-08-16 10:54-0500, Janakarajan Natarajan:
> Enable the Virtual GIF feature. This is done by setting bit 25 at position
> 60h in the vmcb.
> 
> With this feature enabled, the processor uses bit 9 at position 60h as the
> virtual GIF when executing STGI/CLGI instructions.
> 
> Since the execution of STGI by the L1 hypervisor does not cause a return to
> the outermost (L0) hypervisor, the enable_irq_window and enable_nmi_window
> are modified.
> 
> The IRQ and NMI windows will be opened even if GIF is not set, under the
> assumption that on resuming the L1 hypervisor the IRQ and NMI will be
> held pending until the processor executes the STGI instruction.
> 
> Signed-off-by: Janakarajan Natarajan <Janakarajan.Natarajan@....com>
> ---
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> @@ -4702,8 +4735,8 @@ static void enable_nmi_window(struct kvm_vcpu *vcpu)
>  	    == HF_NMI_MASK)
>  		return; /* IRET will cause a vm exit */
>  
> -	if ((svm->vcpu.arch.hflags & HF_GIF_MASK) == 0)
> -		return; /* STGI will cause a vm exit */
> +	if (!vgif_enabled(svm) && !gif_set(svm))
> +		return; /* STGI will cause a vm exit or HW will set VGIF*/

Why don't we enable STGI interception to get notified that the window
has opened?  (I doubt that single stepping would be faster ...)

Thanks.