linux-kernel - Re: [PATCH] KVM: X86: Also reload the debug registers before kvm

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Date:   Fri, 9 Jul 2021 11:49:00 +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] KVM: X86: Also reload the debug registers before
 kvm_x86->run() when the host is using them

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.

Paolo

> For code breakpoint, if the instruction didn't cause vm-exit,
> (for example, the 2nd instruction when kvm emulates instructions
> back to back) emulation.c fails to emulate with DBn.
> 
> For code breakpoint, if the instruction just caused vm-exit.
> It is difficult to analyze this case due to the complex priorities
> between vectored events and fault-like vm-exit.
> Anyway, if it is an instruction that vm-exit has priority over #DB,
> emulation.c fails to emulate with DBn.
> 
> For data breakpoint, a #DB must be delivered to guest or to VMM (when
> guest-debug) after the instruction. But emulation.c doesn't do so.
> 
> And the existence of both of effective DBn (guest debug) and guest DBn
> complicates the problem when we try to emulate them.