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

From: Jan Kiszka <jan.kiszka@siemens.com>
To: Sean Christopherson <seanjc@google.com>,
	Maxim Levitsky <mlevitsk@redhat.com>
Cc: 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>
Subject: Re: [PATCH 2/3] KVM: x86: guest debug: don't inject interrupts while single stepping
Date: Tue, 16 Mar 2021 18:01:36 +0100	[thread overview]
Message-ID: <31c0bba9-0399-1f15-a59b-a8f035e366e8@siemens.com> (raw)
In-Reply-To: <CAMS+r+XFLsFRFLGLaAH3_EnBcxOmyN-XiZqcmKEx2utjNErYsQ@mail.gmail.com>

On 16.03.21 17:50, Sean Christopherson wrote:
> On Tue, Mar 16, 2021, Maxim Levitsky wrote:
>> On Tue, 2021-03-16 at 16:31 +0100, Jan Kiszka wrote:
>>> Back then, when I was hacking on the gdb-stub and KVM support, the
>>> monitor trap flag was not yet broadly available, but the idea to once
>>> use it was already there. Now it can be considered broadly available,
>>> but it would still require some changes to get it in.
>>>
>>> Unfortunately, we don't have such thing with SVM, even recent versions,
>>> right? So, a proper way of avoiding diverting event injections while we
>>> are having the guest in an "incorrect" state should definitely be the goal.
>> Yes, I am not aware of anything like monitor trap on SVM.
>>
>>>
>>> Given that KVM knows whether TF originates solely from guest debugging
>>> or was (also) injected by the guest, we should be able to identify the
>>> cases where your approach is best to apply. And that without any extra
>>> control knob that everyone will only forget to set.
>> Well I think that the downside of this patch is that the user might actually
>> want to single step into an interrupt handler, and this patch makes it a bit
>> more complicated, and changes the default behavior.
> 
> Yes.  And, as is, this also blocks NMIs and SMIs.  I suspect it also doesn't
> prevent weirdness if the guest is running in L2, since IRQs for L1 will cause
> exits from L2 during nested_ops->check_events().
> 
>> I have no objections though to use this patch as is, or at least make this
>> the new default with a new flag to override this.
> 
> That's less bad, but IMO still violates the principle of least surprise, e.g.
> someone that is single-stepping a guest and is expecting an IRQ to fire will be
> all kinds of confused if they see all the proper IRR, ISR, EFLAGS.IF, etc...
> settings, but no interrupt.

From my practical experience with debugging guests via single step,
seeing an interrupt in that case is everything but handy and generally
also not expected (though logical, I agree). IOW: When there is a knob
for it, it will remain off in 99% of the time.

But I see the point of having some control, in an ideal world also an
indication that there are pending events, permitting the user to decide
what to do. But I suspect the gdb frontend and protocol does not easily
permit that.

> 
>> Sean Christopherson, what do you think?
> 
> 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. Would quickly become a mess.

Jan

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