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

[Eric Farman <farman@linux.ibm.com>]

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 :)