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

Eric Farman
2019-06-19T11:12:06Z

On 6/19/19 4:25 AM, Cornelia Huck wrote:
> 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.
>
> I think this is worthwhile.
>
>>
>> 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(-)
>>
>
> Ok, so I just wanted to take a quick look, and then ended up reviewing
> it all :)

Haha, oops! :) Thank you! That was a nice surprise.

>
> Will give others some time to look at this before I queue.
>

Sounds great! I'll get back to my own reviews (notes the gentle
reminder on qemu :)