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Peter Anvin" , Paolo Bonzini , Ingo Molnar Subject: Re: [PATCH 2/3] KVM: x86: guest debug: don't inject interrupts while single stepping Message-ID: References: <20210315221020.661693-3-mlevitsk@redhat.com> <1259724f-1bdb-6229-2772-3192f6d17a4a@siemens.com> <71ae8b75c30fd0f87e760216ad310ddf72d31c7b.camel@redhat.com> <2a44c302-744e-2794-59f6-c921b895726d@siemens.com> <1d27b215a488f8b8fc175e97c5ab973cc811922d.camel@redhat.com> <727e5ef1-f771-1301-88d6-d76f05540b01@siemens.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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?