netdev - Re: [PATCH 3/3] vhost, kcov: collect coverage from vhost

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Date:   Wed, 23 Oct 2019 15:35:12 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     USB list <linux-usb@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        David Windsor <dwindsor@...il.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Anders Roxell <anders.roxell@...aro.org>
Subject: Re: [PATCH 3/3] vhost, kcov: collect coverage from vhost_worker

On Wed, Oct 23, 2019 at 10:36 AM Dmitry Vyukov <dvyukov@...gle.com> wrote:
>
> On Tue, Oct 22, 2019 at 6:46 PM Andrey Konovalov <andreyknvl@...gle.com> wrote:
> >
> > This patch adds kcov_remote_start()/kcov_remote_stop() annotations to the
> > vhost_worker() function, which is responsible for processing vhost works.
> > Since vhost_worker() threads are spawned per vhost device instance
> > the common kcov handle is used for kcov_remote_start()/stop() annotations
> > (see Documentation/dev-tools/kcov.rst for details). As the result kcov can
> > now be used to collect coverage from vhost worker threads.
> >
> > Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
> > ---
> >  drivers/vhost/vhost.c | 6 ++++++
> >  drivers/vhost/vhost.h | 1 +
> >  2 files changed, 7 insertions(+)
> >
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 36ca2cf419bf..a5a557c4b67f 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -30,6 +30,7 @@
> >  #include <linux/sched/signal.h>
> >  #include <linux/interval_tree_generic.h>
> >  #include <linux/nospec.h>
> > +#include <linux/kcov.h>
> >
> >  #include "vhost.h"
> >
> > @@ -357,7 +358,9 @@ static int vhost_worker(void *data)
> >                 llist_for_each_entry_safe(work, work_next, node, node) {
> >                         clear_bit(VHOST_WORK_QUEUED, &work->flags);
> >                         __set_current_state(TASK_RUNNING);
> > +                       kcov_remote_start(dev->kcov_handle);
> >                         work->fn(work);
> > +                       kcov_remote_stop();
> >                         if (need_resched())
> >                                 schedule();
> >                 }
> > @@ -546,6 +549,7 @@ long vhost_dev_set_owner(struct vhost_dev *dev)
> >
> >         /* No owner, become one */
> >         dev->mm = get_task_mm(current);
> > +       dev->kcov_handle = current->kcov_handle;
>
> kcov_handle is not present in task_struct if !CONFIG_KCOV
>
> Also this does not use KCOV_SUBSYSTEM_COMMON.
> We discussed something along the following lines:
>
> u64 kcov_remote_handle(u64 subsys, u64 id)
> {
>   WARN_ON(subsys or id has wrong bits set).

Hm, we can't have warnings in kcov_remote_handle() that is exposed in
uapi headers. What we can do is return 0 (invalid handle) if subsys/id
have incorrect bits set. And then we can either have another
kcov_remote_handle() internally (with a different name though) that
has a warning, or have warning in kcov_remote_start(). WDYT?

>   return ...;
> }
>
> kcov_remote_handle(KCOV_SUBSYSTEM_USB, bus);
> kcov_remote_handle(KCOV_SUBSYSTEM_COMMON, current->kcov_handle);
>
>
> >         worker = kthread_create(vhost_worker, dev, "vhost-%d", current->pid);
> >         if (IS_ERR(worker)) {
> >                 err = PTR_ERR(worker);
> > @@ -571,6 +575,7 @@ long vhost_dev_set_owner(struct vhost_dev *dev)
> >         if (dev->mm)
> >                 mmput(dev->mm);
> >         dev->mm = NULL;
> > +       dev->kcov_handle = 0;
> >  err_mm:
> >         return err;
> >  }
> > @@ -682,6 +687,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> >         if (dev->worker) {
> >                 kthread_stop(dev->worker);
> >                 dev->worker = NULL;
> > +               dev->kcov_handle = 0;
> >         }
> >         if (dev->mm)
> >                 mmput(dev->mm);
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index e9ed2722b633..a123fd70847e 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -173,6 +173,7 @@ struct vhost_dev {
> >         int iov_limit;
> >         int weight;
> >         int byte_weight;
> > +       u64 kcov_handle;
> >  };
> >
> >  bool vhost_exceeds_weight(struct vhost_virtqueue *vq, int pkts, int total_len);
> > --
> > 2.23.0.866.gb869b98d4c-goog
> >