linux-kernel - Re: [PATCH v2 4/5] KVM: selftests: Add a test for KVM

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Date:   Fri, 27 Aug 2021 23:23:30 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     dvhart <dvhart@...radead.org>,
        "Russell King, ARM Linux" <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Guo Ren <guoren@...nel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <hca@...ux.ibm.com>, gor <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        paulmck <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>, shuah <shuah@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-csky <linux-csky@...r.kernel.org>,
        linux-mips <linux-mips@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>,
        linux-kselftest <linux-kselftest@...r.kernel.org>,
        Peter Foley <pefoley@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v2 4/5] KVM: selftests: Add a test for KVM_RUN+rseq to
 detect task migration bugs

On Fri, Aug 27, 2021, Mathieu Desnoyers wrote:
> > So there are effectively three reasons we want a delay:
> > 
> >  1. To allow sched_setaffinity() to coincide with ioctl(KVM_RUN) before KVM can
> >     enter the guest so that the guest doesn't need an arch-specific VM-Exit source.
> > 
> >  2. To let ioctl(KVM_RUN) make its way back to the test before the next round
> >     of migration.
> > 
> >  3. To ensure the read-side can make forward progress, e.g. if sched_getcpu()
> >     involves a syscall.
> > 
> > 
> > After looking at KVM for arm64 and s390, #1 is a bit tenuous because x86 is the
> > only arch that currently uses xfer_to_guest_mode_work(), i.e. the test could be
> > tweaked to be overtly x86-specific.  But since a delay is needed for #2 and #3,
> > I'd prefer to rely on it for #1 as well in the hopes that this test provides
> > coverage for arm64 and/or s390 if they're ever converted to use the common
> > xfer_to_guest_mode_work().
> 
> Now that we have this understanding of why we need the delay, it would be good to
> write this down in a comment within the test.

Ya, I'll get a new version out next week.

> Does it reproduce if we randomize the delay to have it picked randomly from 0us
> to 100us (with 1us step) ? It would remove a lot of the needs for arch-specific
> magic delay value.

My less-than-scientific testing shows that it can reproduce at delays up to ~500us,
but above ~10us the reproducibility starts to drop.  The bug still reproduces
reliably, it just takes more iterations, and obviously the test runs a bit slower.

Any objection to using a 1-10us delay, e.g. a simple usleep((i % 10) + 1)?