linux-kernel - Re: [RFT][PATCH v2 3/9] vfio/ccw: Only pass in contiguous pages

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Message-ID: <13515c3941b0f5d023c68bda83ac9ad6c0ef09e0.camel@linux.ibm.com>
Date:   Fri, 08 Jul 2022 16:25:48 -0400
From:   Eric Farman <farman@...ux.ibm.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Nicolin Chen <nicolinc@...dia.com>
Cc:     kwankhede@...dia.com, corbet@....net, hca@...ux.ibm.com,
        gor@...ux.ibm.com, agordeev@...ux.ibm.com,
        borntraeger@...ux.ibm.com, svens@...ux.ibm.com,
        zhenyuw@...ux.intel.com, zhi.a.wang@...el.com,
        jani.nikula@...ux.intel.com, joonas.lahtinen@...ux.intel.com,
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Subject: Re: [RFT][PATCH v2 3/9] vfio/ccw: Only pass in contiguous pages

On Wed, 2022-07-06 at 14:05 -0300, Jason Gunthorpe wrote:
> On Tue, Jul 05, 2022 at 11:27:53PM -0700, Nicolin Chen wrote:
> > This driver is the only caller of vfio_pin/unpin_pages that might
> > pass
> > in a non-contiguous PFN list, but in many cases it has a contiguous
> > PFN
> > list to process. So letting VFIO API handle a non-contiguous PFN
> > list
> > is actually counterproductive.
> > 
> > Add a pair of simple loops to pass in contiguous PFNs only, to have
> > an
> > efficient implementation in VFIO.
> > 
> > Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> > ---
> >  drivers/s390/cio/vfio_ccw_cp.c | 70 +++++++++++++++++++++++++++---
> > ----
> >  1 file changed, 56 insertions(+), 14 deletions(-)
> 
> I think this is fine as-is for this series, but someone who knows and
> can test ccw should go in and fix things so that pfn_array_alloc()
> doesn't exist. Allocating memory and filling it with consecutive
> integers is kind of silly given we can just call vfio_pin_pages()
> with
> pa_nr directly.
> 
> 	pa->pa_iova_pfn[0] = pa->pa_iova >> PAGE_SHIFT;
> 	pa->pa_pfn[0] = -1ULL;
> 	for (i = 1; i < pa->pa_nr; i++) {
> 		pa->pa_iova_pfn[i] = pa->pa_iova_pfn[i - 1] + 1;
> 
> It looks like only the 'ccw_is_idal' flow can actually create
> non-continuities.

This code is certainly not my favorite, but you're right that it's the
IDAL flow that generates the non-contiguous requests and the code you
reference is simply an initialization for the !IDAL case. As I have a
todo in this code anyway, I'll register your suggestion to see if they
can be untangled.

>  Also the loop in copy_from_iova() should ideally be
> using the much faster 'rw' interface, and not a pin/unpin cycle just
> to memcpy.

I guess I missed when that was added. This looks like low hanging fruit
for some ollllld code regardless of the above. Will get to this once
I'm back. Thank you!

Eric

> 
> If I guess right these changes would significantly speed this driver
> up.
> 
> Anyhow,
> 
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> 
> Jason