svn commit: r269814 - head/sys/dev/xen/blkfront

[John-Mark Gurney]

Roger Pau Monn wrote this message on Tue, Sep 02, 2014 at 11:30 +0200:
> El 29/08/14 a les 19.52, Roger Pau Monné ha escrit:
> > El 28/08/14 a les 20.58, Alexander Motin ha escrit:
> >> On 28.08.2014 21:45, John-Mark Gurney wrote:
> >>> Alexander Motin wrote this message on Thu, Aug 28, 2014 at 21:23 +0300:
> >>>> Hi, Roger.
> >>>>
> >>>> It looks to me like this commit does not work as it should. I got
> >>>> problem when I just tried `newfs /dev/ada0 ; mount /dev/ada0 /mnt`.
> >>>> Somehow newfs does not produce valid filesystem. Problem is reliably
> >>>> repeatable and reverting this commit fixes it.
> >>>>
> >>>> I found at least one possible cause there: If original data buffer is
> >>>> unmapped, misaligned and not physically contiguous, then present x86
> >>>> bus_dmamap_load_bio() implementation will process each physically
> >>>> contiguous segment separately. Due to the misalignment first and last
> >>>> physical segments may have size not multiple to 512 bytes. Since each
> >>>> segment processed separately, they are not joined together, and
> >>>> xbd_queue_cb() is getting segments not multiple to 512 bytes. Attempt to
> >>>> convert them to exact number of sectors in the driver cause data corruption.
> >>>
> >>> Are you sure this isn't a problem w/ the tag not properly specifying
> >>> the correct alignement? 
> >>
> >> I don't know how to specify it stronger then this:
> >>         error = bus_dma_tag_create(
> >>             bus_get_dma_tag(sc->xbd_dev),       /* parent */
> >>             512, PAGE_SIZE,                     /* algnmnt, boundary */
> >>             BUS_SPACE_MAXADDR,                  /* lowaddr */
> >>             BUS_SPACE_MAXADDR,                  /* highaddr */
> >>             NULL, NULL,                         /* filter, filterarg */
> >>             sc->xbd_max_request_size,
> >>             sc->xbd_max_request_segments,
> >>             PAGE_SIZE,                          /* maxsegsize */
> >>             BUS_DMA_ALLOCNOW,                   /* flags */
> >>             busdma_lock_mutex,                  /* lockfunc */
> >>             &sc->xbd_io_lock,                   /* lockarg */
> >>             &sc->xbd_io_dmat);
> >>
> >>> Also, I don't think there is a way for busdma
> >>> to say that you MUST have a segment be a multiple of 512, though you
> >>> could use a 512 boundary, but that would force all segments to only be
> >>> 512 bytes...
> >>
> >> As I understand, that is mandatory requirement for this "hardware".
> >> Alike 4K alignment requirement also exist at least for SDHCI, and IIRC
> >> UHCI/OHCI hardware. Even AHCI requires both segment addresses and
> >> lengths to be even.
> >>
> >> I may be wrong, but I think it is quite likely that hardware that
> >> requires segment address alignment quite likely will have the same
> >> requirements for segments length.
> 
> Hello,
> 
> I have the following fix, which makes sure the total length and the 
> size of each segment is aligned. I'm not very knowledgeable of the 
> busdma code, so someone has to review it.

I feel that this alignment should only be enforced via a new option on
the tag...  I don't see how alignment and segment size should be
conflated...  I could totally see a device that requires an alignement
of 8 bytes, but has a segment size of 16, or vice versa, and requiring
them to be the same means we will bounce unnecesarily...

cc'd scottl since he knows this code better than I... and cperciva as
he touched it for similar reasons..

Oh, I just found PR 152818, where cperciva did a similar fix to
bounce_bus_dmamap_load_buffer for the exact same reason...  It was
committed in r216194...

> ---
> diff --git a/sys/x86/x86/busdma_bounce.c b/sys/x86/x86/busdma_bounce.c
> index d1c75f8..688f559 100644
> --- a/sys/x86/x86/busdma_bounce.c
> +++ b/sys/x86/x86/busdma_bounce.c
> @@ -620,6 +620,8 @@ bounce_bus_dmamap_load_phys(bus_dma_tag_t dmat, bus_dmamap_t map,
>  		segs = dmat->segments;
>  
>  	if ((dmat->bounce_flags & BUS_DMA_COULD_BOUNCE) != 0) {
> +		/* Make sure buflen is aligned */
> +		buflen = roundup2(buflen, dmat->common.alignment);
>  		_bus_dmamap_count_phys(dmat, map, buf, buflen, flags);
>  		if (map->pagesneeded != 0) {
>  			error = _bus_dmamap_reserve_pages(dmat, map, flags);
> @@ -634,6 +636,7 @@ bounce_bus_dmamap_load_phys(bus_dma_tag_t dmat, bus_dmamap_t map,
>  		if (((dmat->bounce_flags & BUS_DMA_COULD_BOUNCE) != 0) &&
>  		    map->pagesneeded != 0 &&
>  		    bus_dma_run_filter(&dmat->common, curaddr)) {
> +			sgsize = roundup2(sgsize, dmat->common.alignment);
>  			sgsize = MIN(sgsize, PAGE_SIZE);
>  			curaddr = add_bounce_page(dmat, map, 0, curaddr,
>  			    sgsize);
> 

Doesn't this same change need to be made to _bus_dmamap_count_phys so that
the cound will be correct?

Also, make sure you review the other arch's bounce implementations, as
they were often copied from the x86 one...

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."