Re: [RFC PATCH v1 0/5] s390: more vfio-ccw code rework

Cornelia Huck
2019-06-21T12:25:43Z

On Tue, 18 Jun 2019 22:23:47 +0200
Eric Farman <farman@linux.ibm.com> wrote:

> A couple little improvements to the malloc load in vfio-ccw.
> Really, there were just (the first) two patches, but then I
> got excited and added a few stylistic ones to the end.
>
> The routine ccwchain_calc_length() has this basic structure:
>
> ccwchain_calc_length
> a0 = kcalloc(CCWCHAIN_LEN_MAX, sizeof(struct ccw1))
> copy_ccw_from_iova(a0, src)
> copy_from_iova
> pfn_array_alloc
> b = kcalloc(len, sizeof(*pa_iova_pfn + *pa_pfn)
> pfn_array_pin
> vfio_pin_pages
> memcpy(a0, src)
> pfn_array_unpin_free
> vfio_unpin_pages
> kfree(b)
> kfree(a0)
>
> We do this EVERY time we process a new channel program chain,
> meaning at least once per SSCH and more if TICs are involved,
> to figure out how many CCWs are chained together. Once that
> is determined, a new piece of memory is allocated (call it a1)
> and then passed to copy_ccw_from_iova() again, but for the
> value calculated by ccwchain_calc_length().
>
> This seems inefficient.
>
> Patch 1 moves the malloc of a0 from the CCW processor to the
> initialization of the device. Since only one SSCH can be
> handled concurrently, we can use this space safely to
> determine how long the chain being processed actually is.
>
> Patch 2 then removes the second copy_ccw_from_iova() call
> entirely, and replaces it with a memcpy from a0 to a1. This
> is done before we process a TIC and thus a second chain, so
> there is no overlap in the storage in channel_program.
>
> Patches 3-5 clean up some things that aren't as clear as I'd
> like, but didn't want to pollute the first two changes.
> For example, patch 3 moves the population of guest_cp to the
> same routine that copies from it, rather than in a called
> function. Meanwhile, patch 4 (and thus, 5) was something I
> had lying around for quite some time, because it looked to
> be structured weird. Maybe that's one bridge too far.
>
> Eric Farman (5):
> vfio-ccw: Move guest_cp storage into common struct
> vfio-ccw: Skip second copy of guest cp to host
> vfio-ccw: Copy CCW data outside length calculation
> vfio-ccw: Factor out the ccw0-to-ccw1 transition
> vfio-ccw: Remove copy_ccw_from_iova()
>
> drivers/s390/cio/vfio_ccw_cp.c | 108 +++++++++++---------------------
> drivers/s390/cio/vfio_ccw_cp.h | 7 +++
> drivers/s390/cio/vfio_ccw_drv.c | 7 +++
> 3 files changed, 52 insertions(+), 70 deletions(-)
>

Thanks, applied.