From: Sean Christopherson <seanjc@google.com>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: dvhart <dvhart@infradead.org>,
"Russell King, ARM Linux" <linux@armlinux.org.uk>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>, Guo Ren <guoren@kernel.org>,
Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
Michael Ellerman <mpe@ellerman.id.au>,
Heiko Carstens <hca@linux.ibm.com>, gor <gor@linux.ibm.com>,
Christian Borntraeger <borntraeger@de.ibm.com>,
rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@redhat.com>,
Oleg Nesterov <oleg@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>,
Andy Lutomirski <luto@kernel.org>, paulmck <paulmck@kernel.org>,
Boqun Feng <boqun.feng@gmail.com>,
Paolo Bonzini <pbonzini@redhat.com>, shuah <shuah@kernel.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
linux-csky <linux-csky@vger.kernel.org>,
linux-mips <linux-mips@vger.kernel.org>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
linux-s390 <linux-s390@vger.kernel.org>,
KVM list <kvm@vger.kernel.org>,
linux-kselftest <linux-kselftest@vger.kernel.org>,
Peter Foley <pefoley@google.com>,
Shakeel Butt <shakeelb@google.com>,
Ben Gardon <bgardon@google.com>
Subject: Re: [PATCH v2 4/5] KVM: selftests: Add a test for KVM_RUN+rseq to detect task migration bugs
Date: Fri, 27 Aug 2021 23:23:30 +0000 [thread overview]
Message-ID: <YSlz8h9SWgeuicak@google.com> (raw)
In-Reply-To: <339641531.29941.1630091374065.JavaMail.zimbra@efficios.com>
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)?
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next prev parent reply other threads:[~2021-08-27 23:26 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-20 22:49 [PATCH v2 0/5] KVM: rseq: Fix and a test for a KVM+rseq bug Sean Christopherson
2021-08-20 22:49 ` [PATCH v2 1/5] KVM: rseq: Update rseq when processing NOTIFY_RESUME on xfer to KVM guest Sean Christopherson
2021-08-23 15:00 ` Mathieu Desnoyers
2021-08-20 22:49 ` [PATCH v2 2/5] entry: rseq: Call rseq_handle_notify_resume() in tracehook_notify_resume() Sean Christopherson
2021-08-20 22:50 ` [PATCH v2 3/5] tools: Move x86 syscall number fallbacks to .../uapi/ Sean Christopherson
2021-08-20 22:50 ` [PATCH v2 4/5] KVM: selftests: Add a test for KVM_RUN+rseq to detect task migration bugs Sean Christopherson
2021-08-23 15:18 ` Mathieu Desnoyers
2021-08-23 15:20 ` Mathieu Desnoyers
2021-08-26 0:51 ` Sean Christopherson
2021-08-26 18:42 ` Mathieu Desnoyers
2021-08-26 23:54 ` Sean Christopherson
2021-08-27 19:09 ` Mathieu Desnoyers
2021-08-27 23:23 ` Sean Christopherson [this message]
2021-08-28 0:06 ` Mathieu Desnoyers
2021-08-20 22:50 ` [PATCH v2 5/5] KVM: selftests: Remove __NR_userfaultfd syscall fallback Sean Christopherson
2021-08-23 23:46 ` Ben Gardon
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