Re: [PATCH v5 1/3] drm/shmem: add support for per object caching flags.

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On 2/27/20 8:53 AM, Gerd Hoffmann wrote:
   Hi,

+		if (!shmem->map_cached)
+			prot = pgprot_writecombine(prot);
   		shmem->vaddr = vmap(shmem->pages, obj->size >> PAGE_SHIFT,
-				    VM_MAP, pgprot_writecombine(PAGE_KERNEL));
+				    VM_MAP, prot)

Wouldn't a vmap with pgprot_writecombine() create conflicting mappings with
the linear kernel map which is not write-combined?
I think so, yes.

Or do you change the linear kernel map of the shmem pages somewhere?
Havn't seen anything doing so while browsing the code.

vmap bypassess at least the
x86 PAT core mapping consistency check and this could potentially cause
spuriously overwritten memory.
Well, I don't think the linear kernel map is ever used to access the
shmem gem objects.  So while this isn't exactly clean it shouldn't
cause problems in practice.

Suggestions how to fix that?

So this has historically caused problems since the linear kernel map has been accessed while prefetching, even if it's never used. Some processors like AMD athlon actually even wrote back the prefetched contents without ever using it.

Also the linear kernel map could be cached somewhere because of the page's previous usage. (hibernation  for example?)

I think it might be safe for some integrated graphics where the driver maintainers can guarantee that it's safe on all particular processors used with that driver, but then IMO it should be moved out to those drivers.

Other drivers needing write-combine shouldn't really use shmem.

So again, to fix the regression, could we revert 0be895893607f ("drm/shmem: switch shmem helper to &drm_gem_object_funcs.mmap") or does that have other implications?

/Thomas





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