Re: [PULL 08/10] vfio-ccw: Introduce a new CRW region

Cornelia Huck
2020-06-02T15:34:47Z

On Tue, 2 Jun 2020 10:46:25 -0400
Eric Farman <farman@linux.ibm.com> wrote:

> On 6/2/20 9:13 AM, Cornelia Huck wrote:
> > On Mon, 25 May 2020 11:41:13 +0200
> > Cornelia Huck <cohuck@redhat.com> wrote:
> >
> >> From: Farhan Ali <alifm@linux.ibm.com>
> >>
> >> This region provides a mechanism to pass a Channel Report Word
> >> that affect vfio-ccw devices, and needs to be passed to the guest
> >> for its awareness and/or processing.
> >>
> >> The base driver (see crw_collect_info()) provides space for two
> >> CRWs, as a subchannel event may have two CRWs chained together
> >> (one for the ssid, one for the subchannel). As vfio-ccw will
> >> deal with everything at the subchannel level, provide space
> >> for a single CRW to be transferred in one shot.
> >>
> >> Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
> >> Signed-off-by: Eric Farman <farman@linux.ibm.com>
> >> Reviewed-by: Cornelia Huck <cohuck@redhat.com>
> >> Message-Id: <20200505122745.53208-7-farman@linux.ibm.com>
> >> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> >> ---
> >> Documentation/s390/vfio-ccw.rst | 19 ++++++++++
> >> drivers/s390/cio/vfio_ccw_chp.c | 55 +++++++++++++++++++++++++++++
> >> drivers/s390/cio/vfio_ccw_drv.c | 20 +++++++++++
> >> drivers/s390/cio/vfio_ccw_ops.c | 8 +++++
> >> drivers/s390/cio/vfio_ccw_private.h | 4 +++
> >> include/uapi/linux/vfio.h | 2 ++
> >> include/uapi/linux/vfio_ccw.h | 8 +++++
> >> 7 files changed, 116 insertions(+)
> >>
> >
> > (...)
> >
> >> @@ -413,6 +423,16 @@ static int __init vfio_ccw_sch_init(void)
> >> goto out_err;
> >> }
> >>
> >> + vfio_ccw_crw_region = kmem_cache_create_usercopy("vfio_ccw_crw_region",
> >> + sizeof(struct ccw_crw_region), 0,
> >> + SLAB_ACCOUNT, 0,
> >> + sizeof(struct ccw_crw_region), NULL);
> >
> > Ugh, I just tested this rebased to the s390 features branch, and I must
> > have used some different options, because I now get
> >
> > kmem_cache_create(vfio_ccw_crw_region) integrity check failed
> >
> > presumably due to the size of the ccw_crw_region.
> >
> > We maybe need to pad it up (leave it unpacked)? Eric, what do you think?
>
> Certainly packing a single one-word struct is weird, and the message is
> coming out of the tiny struct itself:
>
> mm/slab-common.c:88:
> if (!name || in_interrupt() || size < sizeof(void *) ||
> size > KMALLOC_MAX_SIZE) {
> pr_err("kmem_cache_create(%s) integrity check failed\n",
> name);
>
> That's protected by CONFIG_DEBUG_VM which wasn't enabled in my config.
> So playing around with things, we'd have to explicitly add a pad (or the
> second CRW, ha!) to get the struct back up to a doubleword. That'd be
> fine with me.

I think I'll just go with

struct ccw_crw_region {
__u32 crw;
__u32 pad;
} __packed;

here and in the doc. I'll do some tests and do another pull request
tomorrow.