Re: [Intel-gfx] [PATCH v4 3/4] drm/shmem-helpers: Allocate wc pages on x86

["Christian König" <christian.koenig@amd.com>]

Am 13.07.21 um 22:51 schrieb Daniel Vetter:
> intel-gfx-ci realized that something is not quite coherent anymore on
> some platforms for our i915+vgem tests, when I tried to switch vgem
> over to shmem helpers.
>
> After lots of head-scratching I realized that I've removed calls to
> drm_clflush. And we need those. To make this a bit cleaner use the
> same page allocation tooling as ttm, which does internally clflush
> (and more, as neeeded on any platform instead of just the intel x86
> cpus i915 can be combined with).
>
> Unfortunately this doesn't exist on arm, or as a generic feature. For
> that I think only the dma-api can get at wc memory reliably, so maybe
> we'd need some kind of GFP_WC flag to do this properly.

The problem is that this stuff is extremely architecture specific. So 
GFP_WC and GFP_UNCACHED are really what we should aim for in the long term.

And as far as I know we have at least the following possibilities how it 
is implemented:

* A fixed amount of registers which tells the CPU the caching behavior 
for a memory region, e.g. MTRR.
* Some bits of the memory pointers used, e.g. you see the same memory at 
different locations with different caching attributes.
* Some bits in the CPUs page table.
* Some bits in a separate page table.

On top of that there is the PCIe specification which defines non-cache 
snooping access as an extension.

Mixing that with the CPU caching behavior gets you some really nice ways 
to break a driver. In general x86 seems to be rather graceful, but arm 
and PowerPC are easily pissed if you mess that up.

> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Christian König <christian.koenig@amd.com>
> Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Maxime Ripard <mripard@kernel.org>
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Daniel Vetter <daniel@ffwll.ch>

Acked-by: Christian könig <christian.koenig@amd.com>

Regards,
Christian.

> ---
>   drivers/gpu/drm/drm_gem_shmem_helper.c | 14 ++++++++++++++
>   1 file changed, 14 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 296ab1b7c07f..657d2490aaa5 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -10,6 +10,10 @@
>   #include <linux/slab.h>
>   #include <linux/vmalloc.h>
>   
> +#ifdef CONFIG_X86
> +#include <asm/set_memory.h>
> +#endif
> +
>   #include <drm/drm.h>
>   #include <drm/drm_device.h>
>   #include <drm/drm_drv.h>
> @@ -162,6 +166,11 @@ static int drm_gem_shmem_get_pages_locked(struct drm_gem_shmem_object *shmem)
>   		return PTR_ERR(pages);
>   	}
>   
> +#ifdef CONFIG_X86
> +	if (shmem->map_wc)
> +		set_pages_array_wc(pages, obj->size >> PAGE_SHIFT);
> +#endif
> +
>   	shmem->pages = pages;
>   
>   	return 0;
> @@ -203,6 +212,11 @@ static void drm_gem_shmem_put_pages_locked(struct drm_gem_shmem_object *shmem)
>   	if (--shmem->pages_use_count > 0)
>   		return;
>   
> +#ifdef CONFIG_X86
> +	if (shmem->map_wc)
> +		set_pages_array_wb(shmem->pages, obj->size >> PAGE_SHIFT);
> +#endif
> +
>   	drm_gem_put_pages(obj, shmem->pages,
>   			  shmem->pages_mark_dirty_on_put,
>   			  shmem->pages_mark_accessed_on_put);

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