linux-kernel - Re: [PATCH] KVM: X86: Don't reset dr6 unconditionally when the vcpu being scheduled out

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Date:   Tue, 10 Aug 2021 11:59:31 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Lai Jiangshan <laijs@...ux.alibaba.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] KVM: X86: Don't reset dr6 unconditionally when the vcpu
 being scheduled out

On 09/08/21 18:54, Sean Christopherson wrote:
> Not directly related to this patch, but why does KVM_DEBUGREG_RELOAD exist?
> Commit ae561edeb421 ("KVM: x86: DR0-DR3 are not clear on reset") added it to
> ensure DR0-3 are fresh when they're modified through non-standard paths, but I
> don't see any reason why the new values_must_  be loaded into hardware.  eff_db
> needs to be updated, but I don't see why hardware DRs need to be updated unless
> hardware breakpoints are active or DR exiting is disabled, and in those cases
> updating hardware is handled by KVM_DEBUGREG_WONT_EXIT and KVM_DEBUGREG_BP_ENABLED.

The original implementation of KVM_DEBUGREG_WONT_EXIT (by yours truly) 
had a bug where it did not call kvm_update_dr7 and thus 
KVM_DEBUGREG_BP_ENABLED was not set correctly.  I agree that commit 
70e4da7a8ff6 ("KVM: x86: fix root cause for missed hardware 
breakpoints") should have gotten rid of KVM_DEBUGREG_RELOAD altogether.


Paolo