linux-kernel - Re: [PATCH V2 2/3] KVM: X86: Set the hardware DR6 only when KVM_DEBUGREG_WONT

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Date:   Tue, 10 Aug 2021 14:49:54 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org
Subject: Re: [PATCH V2 2/3] KVM: X86: Set the hardware DR6 only when
 KVM_DEBUGREG_WONT_EXIT

On 10/08/21 12:46, Lai Jiangshan wrote:
> I'm OK with it. But I don't think the sketched idea would cause DR6 to 
> be marked uselessly as dirty in SVM. It doesn't mark it dirty if the 
> value is unchanged, and the value is always DR6_ACTIVE_LOW except when 
> it just clears KVM_DEBUGREG_WONT_EXIT.

It would be marked dirty if it is not DR6_ACTIVE_LOW, because it would 
be set first to DR6_ACTIVE_LOW in svm_handle_exit and then set back to 
the guest value on the next entry.

Paolo