linux-kernel - Re: [PATCH 2/3] KVM: x86: guest debug: don't inject interrupts while single stepping

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Message-ID: <d94f9c87-9609-4e7f-04d0-546d27671e51@siemens.com>
Date:   Wed, 17 Mar 2021 10:20:36 +0100
From:   Jan Kiszka <jan.kiszka@...mens.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Wanpeng Li <wanpengli@...cent.com>,
        Kieran Bingham <kbingham@...nel.org>,
        Jessica Yu <jeyu@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Joerg Roedel <joro@...tes.org>,
        Jim Mattson <jmattson@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        Stefano Garzarella <sgarzare@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 2/3] KVM: x86: guest debug: don't inject interrupts while
 single stepping

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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