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

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Message-ID: <CALMp9eQWnUM-O7VmMWTGE2C2YraWxM2K0QcOQnbkctkzg_1pUA@mail.gmail.com>
Date:   Fri, 9 Jul 2021 09:35:30 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.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 Fri, Jul 9, 2021 at 8:52 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> 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.

Data breakpoints are actually quite useful. I/O breakpoints not so much.

> > 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
>