linux-kernel - Re: [PATCH v2 1/5] KVM: nSVM: deal with L1 hypervisor that intercepts interrupts but lets L2 control EFLAGS.IF

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Date:   Mon, 13 Dec 2021 15:29:12 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Cc:     Jim Mattson <jmattson@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2 1/5] KVM: nSVM: deal with L1 hypervisor that
 intercepts interrupts but lets L2 control EFLAGS.IF

On Mon, 2021-12-13 at 14:15 +0100, Paolo Bonzini wrote:
> On 12/13/21 14:07, Maxim Levitsky wrote:
> > > Right, another case is when CLGI is not trapped and the guest therefore
> > > runs with GIF=0.  I think that means that a similar change has to be
> > > done in all the *_allowed functions.
> > 
> > I think that SVM sets real GIF to 1 on VMentry regardless if it is trapped or not.
> 
> Yes, the issue is only when CLGI is not trapped (and vGIF is disabled).

Yes, but I just wanted to clarify that GIF is initially enabled on VM entry
regardless if it is trapped or not, after that the guest can indeed disable
the GIF if CLGI/STGI is not trapped and vGIF disabled.

> 
> > However if not trapped, and neither EFLAGS.IF is trapped, one could enter a guest
> > that has EFLAGS.IF == 0, then the guest could disable GIF, enable EFLAGS.IF,
> > and then enable GIF, but then GIF enablement should trigger out interrupt window
> > VINTR as well.
> 
> While GIF=0 you have svm_nmi_blocked returning true and svm_nmi_allowed 
> returning -EBUSY; that's wrong isn't it?

Yes, 100% agree, patch (and unit test for this as well) is on the way!

Best regards.	
	Maxim Levitsky
> 
> Paolo
>