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Subject: Re: [PATCH] KVM: X86: Also reload the debug registers before
 kvm_x86->run() when the host is using them
To:     Lai Jiangshan <laijs@linux.alibaba.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        linux-kernel@vger.kernel.org
Cc:     Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        kvm@vger.kernel.org
References: <20210628172632.81029-1-jiangshanlai@gmail.com>
 <46e0aaf1-b7cd-288f-e4be-ac59aa04908f@redhat.com>
 <c79d0167-7034-ebe2-97b7-58354d81323d@linux.alibaba.com>
 <397a448e-ffa7-3bea-af86-e92fbb273a07@redhat.com>
 <a4e07fb1-1f36-1078-0695-ff4b72016d48@linux.alibaba.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <01946b5a-9912-3dfb-36f0-031f425432d2@redhat.com>
Date:   Fri, 9 Jul 2021 17:52:09 +0200
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On 09/07/21 12:05, Lai Jiangshan wrote:
> 
> 
> On 2021/7/9 17:49, Paolo Bonzini wrote:
>> On 09/07/21 05:09, Lai Jiangshan wrote:
>>> I just noticed that emulation.c fails to emulate with DBn.
>>> Is there any problem around it?
>>
>> Just what you said, it's not easy and the needs are limited.  I 
>> implemented kvm_vcpu_check_breakpoint because I was interested in 
>> using hardware breakpoints from gdb, even with unrestricted_guest=0 
>> and invalid guest state, but that's it.
> 
> It seems kvm_vcpu_check_breakpoint() handles only for code breakpoint
> and doesn't handle for data breakpoints.

Correct, there's a comment above the call.  But data breakpoint are much 
harder and relatively less useful.

> And no code handles DR7_GD bit when the emulation is not resulted from
> vm-exit. (for example, the non-first instruction when kvm emulates
> instructions back to back and the instruction accesses to DBn).

Good point, that should be fixed too.

Paolo