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Subject: Re: [PATCH 2/3] KVM: x86: guest debug: don't inject interrupts while
 single stepping
To:     Sean Christopherson <seanjc@google.com>
Cc:     Maxim Levitsky <mlevitsk@redhat.com>,
        kvm list <kvm@vger.kernel.org>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Wanpeng Li <wanpengli@tencent.com>,
        Kieran Bingham <kbingham@kernel.org>,
        Jessica Yu <jeyu@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        Joerg Roedel <joro@8bytes.org>,
        Jim Mattson <jmattson@google.com>,
        Borislav Petkov <bp@alien8.de>,
        Stefano Garzarella <sgarzare@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Ingo Molnar <mingo@redhat.com>
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From:   Jan Kiszka <jan.kiszka@siemens.com>
Message-ID: <d94f9c87-9609-4e7f-04d0-546d27671e51@siemens.com>
Date:   Wed, 17 Mar 2021 10:20:36 +0100
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On 16.03.21 18:26, Sean Christopherson wrote:
> On Tue, Mar 16, 2021, Jan Kiszka wrote:
>> On 16.03.21 17:50, Sean Christopherson wrote:
>>> Rather than block all events in KVM, what about having QEMU "pause" the timer?
>>> E.g. save MSR_TSC_DEADLINE and APIC_TMICT (or inspect the guest to find out
>>> which flavor it's using), clear them to zero, then restore both when
>>> single-stepping is disabled.  I think that will work?
>>>
>>
>> No one can stop the clock, and timers are only one source of interrupts.
>> Plus they do not all come from QEMU, some also from KVM or in-kernel
>> sources directly.
> 
> But are any other sources of interrupts a chronic problem?  I 100% agree that

If you are debugging a problem, you are not interested in seening
problems of the debugger, only real ones of your target. IOW: Yes, they
are, even if less likely - for idle VMs.

> this would not be a robust solution, but neither is blocking events in KVM.  At
> least with this approach, the blast radius is somewhat contained.
> 
>> Would quickly become a mess.
> 
> Maybe, but it'd be Qemu's mess ;-)
> 

Nope, it would spread to KVM as well, as indicated above.

Jan

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