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

From: Sean Christopherson <seanjc@google.com>
To: Maxim Levitsky <mlevitsk@redhat.com>
Cc: Jan Kiszka <jan.kiszka@siemens.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>
Subject: Re: [PATCH 2/3] KVM: x86: guest debug: don't inject interrupts while single stepping
Date: Tue, 16 Mar 2021 09:50:23 -0700	[thread overview]
Message-ID: <CAMS+r+XFLsFRFLGLaAH3_EnBcxOmyN-XiZqcmKEx2utjNErYsQ@mail.gmail.com> (raw)
In-Reply-To: <e2cd978e357155dbab21a523bb8981973bd10da7.camel@redhat.com>

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.

> 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?