Re: [PATCH kvm-unit-tests v1 4/6] s390x: smp: Create and use a non-waiting CPU stop

Claudio Imbrenda
2022-03-09T09:27:59Z

On Tue, 08 Mar 2022 16:18:16 -0500
Eric Farman <farman@linux.ibm.com> wrote:

> On Tue, 2022-03-08 at 11:31 +0100, Claudio Imbrenda wrote:
> > On Mon, 07 Mar 2022 14:03:45 -0500
> > Eric Farman <farman@linux.ibm.com> wrote:
> >
> > > On Mon, 2022-03-07 at 16:30 +0100, Claudio Imbrenda wrote:
> > > > On Thu, 3 Mar 2022 22:04:23 +0100
> > > > Eric Farman <farman@linux.ibm.com> wrote:
> > > >
> > > > > When stopping a CPU, kvm-unit-tests serializes/waits for
> > > > > everything
> > > > > to finish, in order to get a consistent result whenever those
> > > > > functions are used.
> > > > >
> > > > > But to test the SIGP STOP itself, these additional measures
> > > > > could
> > > > > mask other problems. For example, did the STOP work, or is the
> > > > > CPU
> > > > > still operating?
> > > > >
> > > > > Let's create a non-waiting SIGP STOP and use it here, to ensure
> > > > > that
> > > > > the CPU is correctly stopped. A smp_cpu_stopped() call will
> > > > > still
> > > > > be used to see that the SIGP STOP has been processed, and the
> > > > > state
> > > > > of the CPU can be used to determine whether the test
> > > > > passes/fails.
> > > > >
> > > > > Signed-off-by: Eric Farman <farman@linux.ibm.com>
> > > > > ---
> > > > > lib/s390x/smp.c | 25 +++++++++++++++++++++++++
> > > > > lib/s390x/smp.h | 1 +
> > > > > s390x/smp.c | 10 ++--------
> > > > > 3 files changed, 28 insertions(+), 8 deletions(-)
> > > > >
> > > > > diff --git a/lib/s390x/smp.c b/lib/s390x/smp.c
> > > > > index 368d6add..84e536e8 100644
> > > > > --- a/lib/s390x/smp.c
> > > > > +++ b/lib/s390x/smp.c
> > > > > @@ -119,6 +119,31 @@ int smp_cpu_stop(uint16_t idx)
> > > > > return rc;
> > > > > }
> > > > >
> > > > > +/*
> > > > > + * Functionally equivalent to smp_cpu_stop(), but without the
> > > > > + * elements that wait/serialize matters itself.
> > > > > + * Used to see if KVM itself is serialized correctly.
> > > > > + */
> > > > > +int smp_cpu_stop_nowait(uint16_t idx)
> > > > > +{
> > > > > + /* refuse to work on the boot CPU */
> > > > > + if (idx == 0)
> > > > > + return -1;
> > > > > +
> > > > > + spin_lock(&lock);
> > > > > +
> > > > > + /* Don't suppress a CC2 with sigp_retry() */
> > > > > + if (smp_sigp(idx, SIGP_STOP, 0, NULL)) {
> > > > > + spin_unlock(&lock);
> > > > > + return -1;
> > > > > + }
> > > > > +
> > > > > + cpus[idx].active = false;
> > > > > + spin_unlock(&lock);
> > > > > +
> > > > > + return 0;
> > > > > +}
> > > > > +
> > > > > int smp_cpu_stop_store_status(uint16_t idx)
> > > > > {
> > > > > int rc;
> > > > > diff --git a/lib/s390x/smp.h b/lib/s390x/smp.h
> > > > > index 1e69a7de..bae03dfd 100644
> > > > > --- a/lib/s390x/smp.h
> > > > > +++ b/lib/s390x/smp.h
> > > > > @@ -44,6 +44,7 @@ bool smp_sense_running_status(uint16_t idx);
> > > > > int smp_cpu_restart(uint16_t idx);
> > > > > int smp_cpu_start(uint16_t idx, struct psw psw);
> > > > > int smp_cpu_stop(uint16_t idx);
> > > > > +int smp_cpu_stop_nowait(uint16_t idx);
> > > > > int smp_cpu_stop_store_status(uint16_t idx);
> > > > > int smp_cpu_destroy(uint16_t idx);
> > > > > int smp_cpu_setup(uint16_t idx, struct psw psw);
> > > > > diff --git a/s390x/smp.c b/s390x/smp.c
> > > > > index 50811bd0..11c2c673 100644
> > > > > --- a/s390x/smp.c
> > > > > +++ b/s390x/smp.c
> > > > > @@ -76,14 +76,8 @@ static void test_restart(void)
> > > > >
> > > > > static void test_stop(void)
> > > > > {
> > > > > - smp_cpu_stop(1);
> > > > > - /*
> > > > > - * The smp library waits for the CPU to shut down, but
> > > > > let's
> > > > > - * also do it here, so we don't rely on the library
> > > > > - * implementation
> > > > > - */
> > > > > - while (!smp_cpu_stopped(1)) {}
> > > > > - report_pass("stop");
> > > > > + smp_cpu_stop_nowait(1);
> > > >
> > > > can it happen that the SIGP STOP order is accepted, but the
> > > > target
> > > > CPU
> > > > is still running (and not even busy)?
> > >
> > > Of course. A SIGP that's processed by userspace (which is many of
> > > them)
> > > injects a STOP IRQ back to the kernel, which means the CPU might
> > > not be
> > > stopped for some time. But...
> > >
> > > >
> > > > > + report(smp_cpu_stopped(1), "stop");
> > > >
> > > > e.g. can this ^ check race with the actual stopping of the CPU?
> > >
> > > ...the smp_cpu_stopped() routine now loops on the CC2 that SIGP
> > > SENSE
> > > returns because of that pending IRQ. If SIGP SENSE returns CC0/1,
> > > then
> > > the CPU can correctly be identified stopped/operating, and the test
> > > can
> > > correctly pass/fail.
> >
> > my question was: is it possible architecturally that there is a
> > window
> > where the STOP order is accepted, but a SENSE on the target CPU still
> > successfully returns that the CPU is running?
>
> Not to my knowledge. The "Conditions Determining Response" section of
> POPS (v12, page 4-95; also below) specifically states that the SIGP
> SENSE will return CC2 when a SIGP STOP order is outstanding.
>
> In our implementation, the stop IRQ will be injected before the STOP
> order gets accepted, such that a SIGP SENSE would see it immediately.
>
> """
> A previously issued start, stop, restart, stop-
> and-store-status, set-prefix, store-status-at-
> address order, or store-additional-status-at-
> address has been accepted by the
> addressed CPU, and execution of the func-
> tion requested by the order has not yet been
> completed.
> ...
> If the currently specified order is sense, external
> call, emergency signal, start, stop, restart, stop
> and store status, set prefix, store status at
> address, set architecture, set multithreading, or
> store additional status at address, then the order
> is rejected, and condition code 2 is set.
> """
>
> >
> > in other words: is it specified architecturally that, once an order
> > is
> > accepted for a target CPU, that CPU can't accept any other order (and
> > will return CC2), including SENSE, until the order has been completed
> > successfully?
>
> The POPS quote I placed above excludes store-extended-status-address,
> conditional-emergency-signal, and sense-running-status orders as
> returning a CC2. That's something Janosch and I have chatted about
> offline, but don't have an answer to at this time. Besides that, any
> other order would get a CC2 until the STOP has been completed.

this is what I wanted to know, thanks

you can add

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

>
> >
> > > >
> > > > > }
> > > > >
> > > > > static void test_stop_store_status(void)
>