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

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Message-ID: <CALMp9eRrPtidJOamfSnkHT-PnPRXZ7SXkw9JFW-Q38v0Q-ws5w@mail.gmail.com>
Date:   Fri, 9 Jul 2021 10:01:54 -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 9:59 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 09/07/21 18:35, Jim Mattson wrote:
> >>>> 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.
>
> Normally yes; much less for the specific case of debugging
> invalid-guest-state or other invocations of the emulator.

Agreed. But if they don't actually work in-guest (because the emulator
ignores them), then their normal usefulness is curtailed.