On Mon, Dec 07, 2020 at 02:12:26PM +0800, Ying-Tsun Huang wrote:
> In mtrr_type_lookup, if the input memory address region is not in the
> MTRR, over 4GB, and not over the top of memory, write-back attribute
> is returned. These condition checks are for ensuring the input memory
> address region is mapped to the physical memory actually.
>
> However, if the end address is just aligned with the top of memory,
> the condition check treats the address is over the top of memory, and
> write-back attribute is not returned.
Oh fun. So to make sure I understand this correctly end ends up equal to
TOM2?
> There is a real case of NVDIMM. The nd_pmem module tries to map
> NVDIMMs as cacheable memories when NVDIMMs are connected. If a NVDIMM
> is the last of the DIMMs, the performance of this NVDIMM becomes very
> low since it aligned with the top of memory and its memory type is
> uncached-minus.
>
> To check the top of memory should use "<=" instead of "<" since both the
> input end address and the value of top of memory are actually the start
> of next region.
Right, so looking at that function, it calls
mtrr_type_lookup_variable(), among others, and that does:
/* Make end inclusive instead of exclusive */
end--;
which sounds to me like it expects ranges with exclusive end.
So maybe it would be better to do something like:
/*
* Blurb about end address being == tom2, perhaps give your example
*/
end--;
above the check so that it is absolutely obvious why this is done.
But but, looking at this more, the PPR says about TOM2:
"This value is normally placed above 4G. From 4G to TOM2 - 1 is DRAM;
TOM2 and above is MMIO."
So the check is *actually* correct - TOM2 - 1 is DRAM so you need '<'.
Unless you do end-- before, which would make sense and suggest the end
decrement to be the proper fix.
Hmm?
> Fixes: b73522e0c1be ("x86/mm/mtrr: Enhance MTRR checks in kernel mapping
> helpers")
I think you mean:
35605a1027ac ("x86: enable PAT for amd k8 and fam10h")
which first added that check.
Btw, while doing git archeology, I saw that mtrr_type_lookup() used to do end--
on function entry, see
2e5d9c857d4e ("x86: PAT infrastructure patch")
Thx.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
