Re: [PATCH 6/7] s390/cio: Don't pin vfio pages for empty transfers

[Cornelia Huck <cohuck@redhat.com>]

On Fri,  3 May 2019 15:49:11 +0200
Eric Farman <farman@linux.ibm.com> wrote:

> If a CCW has a count of zero, then no data will be transferred and
> pinning/unpinning memory is unnecessary.
> 
> In addition to that, the skip flag of a CCW offers the possibility of
> data not being transferred, but is only meaningful for certain commands.
> Specifically, it is only applicable for a read, read backward, sense, or
> sense ID CCW and will be ignored for any other command code
> (SA22-7832-11 page 15-64, and figure 15-30 on page 15-75).

This made me look at QEMU, and it seems that we cheerfully ignore that
flag so far in our ccw interpretation code :/

> 
> (A sense ID is xE4, while a sense is x04 with possible modifiers in the
> upper four bits.  So we will cover the whole "family" of sense CCWs.)
> 
> For all those scenarios, since there is no requirement for the target
> address to be valid, we should skip the call to vfio_pin_pages() and
> rely on the IDAL address we have allocated/built for the channel
> program.  The fact that the individual IDAWs within the IDAL are
> invalid is fine, since they aren't actually checked in these cases.
> 
> Set pa_nr to zero, when skipping the pfn_array_pin() call, since it is
> defined as the number of pages pinned.  This will cause the vfio unpin
> logic to return -EINVAL, but since the return code is not checked it
> will not harm our cleanup path.

We could also try to skip the unpinning, but this works as well.

> 
> As we do this, since the pfn_array_pin() routine returns the number of
> pages pinned, and we might not be doing that, the logic for converting
> a CCW from direct-addressed to IDAL needs to ensure there is room for
> one IDAW in the IDAL being built since a zero-length IDAL isn't great.
> 
> Signed-off-by: Eric Farman <farman@linux.ibm.com>
> ---
>  drivers/s390/cio/vfio_ccw_cp.c | 61 +++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 55 insertions(+), 6 deletions(-)

Looks good to me.