On Wed, Jan 17, 2018 at 12:39 AM, Daniel Vetter daniel@ffwll.ch wrote:
On Tue, Jan 16, 2018 at 04:35:59PM -0800, Gurchetan Singh wrote:
This is required to use buffers allocated by vgem on AMD and ARM devices. We're experiencing a case where eviction of the cache races with
userspace
writes. To fix this, flush the cache after retrieving a page.
Signed-off-by: Gurchetan Singh gurchetansingh@chromium.org
drivers/gpu/drm/vgem/vgem_drv.c | 1 + 1 file changed, 1 insertion(+)
diff --git a/drivers/gpu/drm/vgem/vgem_drv.c
b/drivers/gpu/drm/vgem/vgem_drv.c
index 35bfdfb746a7..fb263969f02d 100644 --- a/drivers/gpu/drm/vgem/vgem_drv.c +++ b/drivers/gpu/drm/vgem/vgem_drv.c @@ -112,6 +112,7 @@ static int vgem_gem_fault(struct vm_fault *vmf) break; }
drm_flush_pages(obj->base.dev->dev, &page, 1);Uh ... what exactly are you doing?
Asking because the entire "who's responsible for coherency" story is entirely undefined still when doing buffer sharing :-/ What is clear is that currently vgem entirely ignores this (there's not begin/end_cpu_access callback), mostly because the shared dma-buf support in drm_prime.c also entirely ignores this.
This patch isn't trying to address the case of a dma-buf imported into vgem. It's trying to address the case when a buffer is created by vgem_gem_dumb_create, mapped by vgem_gem_dumb_map and then accessed by user space. Since the page retrieved by shmem_read_mapping_page during the page fault may still be in the cache, we're experiencing incorrect data in buffer. Here's the test case we're running:
https://chromium.googlesource.com/chromiumos/platform/drm-tests/+/master/vge...
It fails on line 210 on AMD and ARM devices (not Intel though).
And doing a one-time only
flushing in your fault handler is definitely not going to fix this (at least not if you do anything else than one-shot uploads).
There used to be a be vgem_gem_get_pages function, but that's been removed. I don't know where else to flush in this situation.
-Daniel
} return ret;}
2.13.5
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-- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch