Re: [PATCH 1/2] drm: add cache support for arm64

Rob Clark
2019-08-07T17:30:17Z

On Wed, Aug 7, 2019 at 9:50 AM Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Wed, Aug 07, 2019 at 09:15:54AM -0700, Rob Clark wrote:
> > On Wed, Aug 7, 2019 at 5:38 AM Mark Rutland <mark.rutland@arm.com> wrote:
> > >
> > > On Tue, Aug 06, 2019 at 09:31:55AM -0700, Rob Clark wrote:
> > > > On Tue, Aug 6, 2019 at 7:35 AM Mark Rutland <mark.rutland@arm.com> wrote:
> > > > >
> > > > > On Tue, Aug 06, 2019 at 07:11:41AM -0700, Rob Clark wrote:
> > > > > > On Tue, Aug 6, 2019 at 1:48 AM Christoph Hellwig <hch@lst.de> wrote:
> > > > > > >
> > > > > > > This goes in the wrong direction. drm_cflush_* are a bad API we need to
> > > > > > > get rid of, not add use of it. The reason for that is two-fold:
> > > > > > >
> > > > > > > a) it doesn't address how cache maintaince actually works in most
> > > > > > > platforms. When talking about a cache we three fundamental operations:
> > > > > > >
> > > > > > > 1) write back - this writes the content of the cache back to the
> > > > > > > backing memory
> > > > > > > 2) invalidate - this remove the content of the cache
> > > > > > > 3) write back + invalidate - do both of the above
> > > > > >
> > > > > > Agreed that drm_cflush_* isn't a great API. In this particular case
> > > > > > (IIUC), I need wb+inv so that there aren't dirty cache lines that drop
> > > > > > out to memory later, and so that I don't get a cache hit on
> > > > > > uncached/wc mmap'ing.
> > > > >
> > > > > Is there a cacheable alias lying around (e.g. the linear map), or are
> > > > > these addresses only mapped uncached/wc?
> > > > >
> > > > > If there's a cacheable alias, performing an invalidate isn't sufficient,
> > > > > since a CPU can allocate a new (clean) entry at any point in time (e.g.
> > > > > as a result of prefetching or arbitrary speculation).
> > > >
> > > > I *believe* that there are not alias mappings (that I don't control
> > > > myself) for pages coming from
> > > > shmem_file_setup()/shmem_read_mapping_page()..
> > >
> > > AFAICT, that's regular anonymous memory, so there will be a cacheable
> > > alias in the linear/direct map.
> >
> > tbh, I'm not 100% sure whether there is a cacheable alias, or whether
> > any potential linear map is torn down.
>
> I'm fairly confident that the linear/direct map cacheable alias is not
> torn down when pages are allocated. The gneeric page allocation code
> doesn't do so, and I see nothing the shmem code to do so.
>
> For arm64, we can tear down portions of the linear map, but that has to
> be done explicitly, and this is only possible when using rodata_full. If
> not using rodata_full, it is not possible to dynamically tear down the
> cacheable alias.

So, we do end up using GFP_HIGHUSER, which appears to get passed thru
when shmem gets to the point of actually allocating pages.. not sure
if that just ends up being a hint, or if it guarantees that we don't
get something in the linear map.

(Bear with me while I "page" this all back in.. last time I dug thru
the shmem code was probably pre-armv8, or at least before I had any
armv8 hw)

BR,
-R