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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)?

  reply	other threads:[~2021-08-27 23:23 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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